It does bug me when someone completely random slams into me after a race, but if it's the racer I just swapped positions with for the past 5 laps, it's fair game. :P Besides that, there's no way to stop this, and it's not really a problem in the first place. If a majority of people thought it was something to be banned for, then you have your wish, but most of the time it's just playing around and you were unlucky.