We frequently see unintentional "cheating" on segment leaderboards by users who leave Strava recording running while driving home after an activity. While it's impossible to automatically flag all activities recorded using motor vehicles, it should be practical to catch those who travel some distance on freeways where bikes and pedestrians aren't allowed. Check OpenStreetMap for feature type motorway. These are always controlled access roads, known as freeways in the USA.
There are a lot of other good ideas in this forum for automatic flagging based on unrealistic speeds or lack of supplementary sensor data, but this idea is specifically about detecting travel on freeways which can't possibly represent a real sport activity.
