First of all, flagging can only be done from the desktop version of Strava.  It is not available at this time for the mobile app.  
If you see an activity that you believe is not correct for whatever reason, including impossible speeds, you should flag it.  Strava relies on other users to help clean up the leaderboards.  When you open the activity on a desktop browser, you will see a menu on the left side of the activity details.  At the bottom, click on the three dots.  You will see another menu come up with the top option being "Flag".  Click on this to open the activity flagging page.  On this page, you must select from the 4 available "reasons" for flagging the activity, "Activity was in a vehicle", "Wrong activity type", "Bad GPS data", and "Other".  Once you have selected the option that best associates with why you want to flag the activity (for impossible speeds, it is likely either "Activity was in a vehicle" or "Bad GPS data"), you then need to manually type in a description of why you feel this applies.  Essentially, describe what in the activity file made you sure it was unreasonably fast.  Once you are done, click the "Flag" button on the bottom left of the page.  At this point, the activity has been flagged and will be removed from the leaderboards.  The person will have an opportunity to review the flag and correct the activity or dispute the flag, but that is another subject.  

Here is a reference article from Strava describing the process: The Activity Flag: Remove an Activity's segment results from the Leaderboard – Strava Support

Per the reference article on strava help:

1. 1. Find the bad segment match: When you find an incorrect or improper segment time, click through to the Strava Activity page. If you're on a segment leaderboard, you will need to click on the date of the segment result to reach the activity.
   2. Flag the activity and leave a detailed description of the problem: If the activity should be flagged, use the ellipses icon at the left side of the page to select Flag. Select the correct reason for the flag and leave descriptive and thorough comments. Leaving detailed comments is incredibly important for the Support Team to be able to appropriately address the flag, so please be as descriptive and specific as possible. Naming affected segments on the activity, specifying the location of a GPS data problem, and relaying personal knowledge about the area or the athlete are all valuable to include.
      1. If the activity appears to have data that was done by a motorized vehicle or partially done by a motorized vehicle, select the first option. 
      2. For activities that appear to be listed as the wrong activity type, like a ride that is accidentally categorized as a run, select the second option. 
      3. If there is bad GPS data that causes inaccurate mileage or segment times, select "Bad GPS data". For specific segment errors, it is helpful if you include which segments are affected in the comments section. 
      4. For other issues that are not listed here, select "Other" and leave a thorough description of the problem in the comment field. 

   3. When you're done, press the "Flag" button. A yellow bar will appear at the top of the activity page, signifying that the activity has been flagged. The activity will be automatically removed from all segment leaderboards. The activity remains on the site, and the segments remain on the activity page with achievements; however, all the public results are removed from the segment leaderboards. 

HOW TO FLAG 

1. Read the Strava Support article
2. On desktop, click through to the activity with the problematic segment attempt.
3. Click the ellipses to open the little menu and click "Flag"
4. Select or enter the reason for flagging.
5. Click "Flag" again.

That's it! 🙂

Question: Someone rode a long segment at an impossible speed! How can I flag an incorrect activity?

Key points: riding, impossible speed, flagging incorrect activity

When it comes to riding at an impossible speed, it's likely that a user uploads an activity using data that is motor-assisted either by riding an electric bike (or a non-traditional bicycle with full fairings or aerodynamic modifications including velomobiles), driving a car or motorcycle, or cycling behind a vehicle (motor-paced rides). It's also likely that the GPS data is being manipulated. Such actions may affect the fairness and accuracy of Strava leaderboards.

Strava lets the community control a Segment Leaderboard's accuracy by offering the flag tool. Hence, here are the steps to flag incorrect activity through Strava website, following Strava's flagging guidelines are as follows:

1. Find the bad segment match: When you find an incorrect or improper segment time, click through to the Strava Activity page. If you're on a segment leaderboard, you will need to click on the date of the segment result to reach the activity.
2. Flag the activity and leave a detailed description of the problem: If the activity should be flagged, use the ellipses icon at the left side of the page to select Flag. Select the correct reason for the flag and leave descriptive and thorough comments. Leaving detailed comments is incredibly important for the Support Team to be able to appropriately address the flag, so please be as descriptive and specific as possible. Naming affected segments on the activity, specifying the location of a GPS data problem, and relaying personal knowledge about the area or the athlete are all valuable to include.
   1. If the activity appears to have data that was done by a motorized vehicle or partially done by a motorized vehicle, select the first option. 
   2. For activities that appear to be listed as the wrong activity type, like a ride that is accidentally categorized as a run, select the second option. 
   3. If there is bad GPS data that causes inaccurate mileage or segment times, select "Bad GPS data". For specific segment errors, it is helpful if you include which segments are affected in the comments section. 
   4. For other issues that are not listed here, select "Other" and leave a thorough description of the problem in the comment field. 
3. When you're done, press the "Flag" button. A yellow bar will appear at the top of the activity page, signifying that the activity has been flagged. The activity will be automatically removed from all segment leaderboards. The activity remains on the site, and the segments remain on the activity page with achievements; however, all the public results are removed from the segment leaderboards. 

References:

The Activity Flag: Remove an Activity's segment results from the Leaderboard

Segment Leaderboard Guidelines

Reporting Content on Strava for Violations of Our Community Standards

Thank you to those who participated and congratulations to @anchskier the winner of this round of Wisdom Wednesday!

To learn more about activity flags, check out these support articles:

How to Report Cheating on Strava 

The Activity Flag: Remove an Activity's segment results from the Leaderboard