I would do something like this: you get a certain amount of complaints, you are forced to race as a ghost and, whenever you're not in last position, your car will apply full brakes until you get passed by everyone. This way you'll get humiliated by finishing last no matter what you do. The first time this will apply for 10 races, the second time 20, etc.. Oh, and you'll have to actually finish the race, It won't be counted until you are back in the lobby after the race. Obviously this system would require a dedicated staff checking replays to make sure the function is not abused by trolls.