You would only put a water mark on it if you where uploading the flat texture to the forum as a preview, so you have a copy of the texture without a watermark then make one with a watermark to show as an example.
Just like if you have art that you are selling(Wartermark is not a toribash thing it is just an anti-fraud method) you would probably have a copy of the art with a watermark on it.
So to answer your question
if you're making a texture for yourself, and activate it, how do you prevent people from accusing you of stealing the texture?
You do not wear the copy with a watermark on it.
hope this helped, ~Tuna