IF you can't race cleanly, don't race with people you do not know.
If you REALLY find that dirty racers are ruining the game, find some friends to play with, rather than depending on Matchmaking.
While I hate to say that there are simply people that need to win at all costs, or prevent other people from enjoying the game, these types of people exist, and you just literally have to deal with them... in every game.
There is simply no way to police the situation 24/7, nor is there a way to tell someone that they cannot play your game any more, that they paid money to do, simply because your definition of fun, differs from their definition.
There is a minimum of griefers in Forza, and when you do run into one, do what you are supposed to do. AVOID their gamertag, report them for unsporting conduct, and find a new lobby.