Yeah I come across this problem a lot, since I make my games from scratch these days. Really the only way around it is to invent your own file formats and use those. That way to use the assets, people have to reverse engineer the game first, which can be very difficult.
Viewing post in Protecting game assets
That seems to be the general consensus I was seeing online as well. That or some sort of encryption. Both solutions seemed a bit complicated.
One of the asset websites I was looking at had this in their FAQs:
"Do I need to protect the items within my end product from being re-used?
You should not permit end users of the end product to extract an item from the end product. You should do this by technological means if feasible, or by other means, like in the user terms for your end product."