How, exactly, do you propose we make this enforceable? The textures you use are just image files that need to be available for people to see your textures at all. You necessarily cannot simultaneously have "the ability to see a person's textures in game" and "the inability to copy a person's texture files". The former means the texture has been read and stored in memory, while the latter precisely means that the former is impossible.
If you're not using the textures in game, and you're concerned about people stealing them, then just don't upload them in the first place.