Josh, on the Lets Talk Guild forum registration is not automatic. A person desiring "admission" must e-mail the owner to request inclusion. That requires a brief statement about why the person wants to join the forum. If approved, then the owner lets the person in. So there may be a couple-day waiting period before admission is granted.
Not foolproof, and more work for you, but it seems to work well there to keep out most of the riffraff.