Sounds like Joe's got it handled as well as can be expected.
ISPs are not a reliable way of telling who's posting or whether posts came from the same person.
I know it's hard to make sure a "Guest" uses the same handle every time, but at least you should have to use SOME sort of user name when you post.   And there's really not much reason for a "Guest" NOT to stick to a consistent name (e.g., curmudgeon44) at least within a particular thread.