First post ever!

Treat them like spam (make sure they're in your spam filters). I work at a public service point (I'm a reference librarian at a university) and can tell you that any time you interface with the public "for free" in ANY context, you are guaranteed to get a nutcase now and then. It's probabilistic. In fact, the MORE popular your free stuff is, the MORE likely this will happen.

Take it as a badge of honor (then TOTALLY ignore the twit).