Be specific, please. What do you want your license to allow people to do?
You probably want a formulation along the lines of, "you can use these assets without limitation as part of your games, but you're not allowed to redistribute the asset packs themselves, in whole or in part, in any medium and for any purpose". It's pretty common.
Oh, obligatory disclaimer: I am not a lawyer. Talk to a real one for a formulation that can hold water in court.