[quote name='WhipSmartBanky']If they're playing on the same console, all profiles on that 360 have access to the content that was purchased on that console.[/QUOTE]
Exactly what I was thinking.... the other option is to... well, buy a new 360, don't do a data transfer of the games (excluding saves if you want) and then re-download the content through your own account settings. Expensive solution, but it should work if you want to shell out the cash.
Other than this, you might want to try the parental controls. Haven't really browsed it before, but it might have an option to disable some things. According to the Xbox Parental Control page on the main xbox site, you can disable certain games based on rating.