First I would make sure it is in fact on your land and not just close to the border of a property. You didn't mention if it was in the heart of your propery or close to a line. I have a surveyor friend of mine and you'd be surprised how many people think they own land they in fact do not.....

Then if it is in fact on your land, I would notify the person who built and ask why? It could be an honest mistake as I mentioned above in not knowing where the appropriate line is...he may offer to tear it down and move it to his property or if he's rude, then follow up on a call to aurthorities which are usually a joke, tear the stand down or use it yourself, also placing posters on and around the stand could help you in prosecution.

Not a good situation but you better make sure your correct in your assumptions...remember the laws protect the guilty, not the innocent.