Honestly, you are going to have this happen no matter what. It's happened in both Morrowind and Oblivion, and I'm sure (but don't have firsthand experience) it happens in all other modding communities. The people that are dishonest enough to abuse your permission would probably be dishonest enough to use your stuff even without your permission.

However, for the most part good people control most of the mod distribution. I know you can get someone to take down a mod that uses your artwork without permission from sites like PES and Nexus. Most of the community would come to your defense if that happens. But still, once you release anything, whether you give permission or not to reuse your stuff, somebody is going to ignore it.

The only solution is to release nothing, and what really svcks is some artists have been so hurt and angered by abuse of their stuff that they end up doing just that - cancelling all further sharing with the community and pulling all their mods. I hate it when bad apples drive our good community members to that extreme.

EDIT: I wanted to clarify that of course the MAJORITY of our community would never do that. But there are always a few bad apples.