Air gap. If you don’t want content to be used without your permission, it never leaves your computer. This is the only protection that works.

If you want others to see your content, however, you have to accept some degree of trade off with it being misappropriated. Blatant cases can be addressed the same as they always were, but a model overfitting to your original work poses an interesting question for which I’m not aware of any legal precedents having been set yet.