For Ferrari, if you break their stipulations, they put you on a blacklist and won't sell you another ever again. I can't find any other hard-and-fast things they do because there's a lot of rumor milling, but barring you from purchases and ending your dealership maintenance seem to be "for sure". I imagine it comes with some other stigmas from the community too. But much like real estate covenants if you agree to something in a contract, and then break it, you're subject to civil action. In a Covenant, the contract holders are permitted to buy back your house and evict you.

Deadmau5 was threatened with a lawsuit over his "Purrari" Nyan cat livery.