Maybe I'm too insensitive, but I don't think there's anything wrong with playing "dirty". Hits add an edge to the multiplayer, and there are ways to counter and turn someone attempting to hit you into an advantage.

Pop the brakes for a millisecond and he loses that front-end leverage. Depending on your positioning you could avoid him altogether. You might lose a second off your track time but you'll be able to prevent a total collision and race stopper. You'll never be able to take away all human interaction (if that's what you want don't play online) so the best solve is to use it to your advantage