I see the same thing - if it's a bike Segment I have ridden then under the Actions button I have 2 options in "Tell us about this Segment" - either I can report an inappropriate name or I can report the Segment as hazardous.  However, if I have not ridden it I can only flag an inappropriate name.  This is a bit frustrating as, like you, I occasionally see Segments that are hazardous (e.g. Segment goes through a busy intersection) but I can only report it if I have attempted the Segment.

@Andy1 and @Ian11, the hazardous flagging tool is designed so that people who complete the segment can attest to their experience. If you’re unable to safely complete the segment, you can submit a ticket to our support team for review. Please include relevant details in your ticket submission such as the segment link, an explanation of why you believe the segment is hazardous, and any additional supporting resources.

Why can't we flag a segment as being 'illegal' as it runs over a path where cycling is not permitted without having to complete it? This makes no sense whatsoever as I would essentially have to break the law to complete the segment!
I can see a number of segments in my local area that run on footpaths where cycling is prohibited, all I want to do is get them removed for everybody's advantage, nobody wants to be riding trails they shouldn't be but strava seems to actively encourage it by not giving any tools to flag segments that are inappropriate.  

Another thing to consider is that in some cases, there could be special events that allow biking on trails that are closed to bikes otherwise.  I participate in a race every winter that is held on trails that are closed to bikes at all times outside of the race itself.  It is fun because it is the only time we are allowed to bike these trails.  I know that is only something that happens fairly infrequently, but you have to take that into account when you are looking at segments on "closed" trails.  There may have been a time or event that allowed them at some point.  

@AGarland - One way to work around that problem is to run the segment, but then edit your "run" activity to a "ride" just long enough to see and flag the segment.  You can then change your activity back to a "run" once you are done with the flagging process.  Not quite the solution you are looking for, but one way you can do it without breaking the law and actually riding the segment in question.

Thanks @anchskier I didn't know you could do that. 

Although as you alluded to my point is that there should be a mechanism to allow members to flags segments that they know have an issue regardless of whether they have completed it or not.  As i see it showing 'illegal' segments in the heatmap is just encouraging people to ride them and cause aggravation with the locals. 

I see this also as a big problem of strava.
There are all over the map illegal trails, strava is best known for discovering illegal trails in the bike community.
The only reason I use the strava map is to discover illegal trails. 
Strava should be aware, that this topic is growing with the increasing popularity of mtb and strava will mostlikely get negative headlines as soon this topic starts to be political. 
I tried to flag some illegal trails, had success in the first attempt, as I already have ridden the trails.
But now the segments are reuploaded again by other users and the yshow up as before. Ridiculous. 

Please Strava implement features to suppress illegal trails as segments and also from the heatmap! Trailforks is doing a fantastic job with hidden trails to protect sensitive areas and not encouraging people to ride it.