Apart from JJay's link, here's some suggestions on reporting a user
1. Record the accident (by any software you like, windows 10 can use the win+G to record)
2. Upload it (to youtube or whatever you want)
3. Mark the time and user id of the evidence (I suggest using Youtube as they provide a description box where you can mark them down, and the timestamp function can help moderator jump to that specific time faster)
(User ID gain from the in-game command /pinfo)
(Can also search from C:\Users\[Your username]\Documents\ETS2MP\logs\log_spawning_YYYY.MM.DD.log)
5. Continue the reporting on JJay's link
6.Wait for the moderators to process the report , the load of the report system are in a very high condition right now, so it may take weeks (One eternity later) for a result)
moderator to process