A little more information could help.  There are a couple ways depending on what happened.  If it was a situation where you accidentally left the recording going after you were done riding and included some driving in a car, you can go back and use the “crop” tool in the edit menu to remove the section that was incorrectly recorded. If it was something in the middle of a ride that incorrectly matched a segment, that is more difficult since there isn’t a way to just remove a segment match.  You could make the activity visible to “only you” or “followers” so that none of the ride will show in in segment leaderboards.  If you can post a link to the activity and point out which segment has the issue, it might be easier to give some ideas of how to address the issue.

Hello, thanks for your ideas. The KOM segment is in the middle of my ride, never had this in the last years before. I will delete this activity...Thanks for help...Oliver

No need to completely delete the activity.  You could just make it visible to you so you don’t lose credit for doing the ride.  That will remove it from the leaderboards so you don’t have that issue hanging out there.  Another option would be to use the “split” tool to cut out that portion in the middle of the ride and leave the rest of the ride (before and after) as two activities.  If you need help with those options, just let me know and I can walk you through the steps.

O.k. now it's only visible for followers and the KOM is no more on the leaderboard, thanks for help bye bye