I think others have suggested something along the same lines of your suggestions. I like to keep the country set to United States, to make it easier for my students. The spammers on my system have email accounts from aol.com, and hotmail.com. But so do some of my valid users.
All these spammers don't make it into any course, which is why I want to filter on this condition. Ideally, I would prefer a filter that states "Delete users not in any course after xx days."