At the moment i have a list of hundreds of segments in Garmin connect, most of them i have bin only once.
How can i remove this "once in a lifetime segments" so i have a proper overview of mine segments that realy matters?
Why don't you just use the filter "Segments that I created" to view those?
Otherwise I think you cannot remove yourself from segments created by others without deleting the respective activity.
I discovered you can use "trim" under settings gear in the upper right of connect to remove distance or time from the beginning or the end of a run and save it, then export back to your Garmin watch assuming you wanted it correct in both places.