How can he stop the guy? What's to stop him from creating new identities and signing in? It seems you can only hope that he gets burned out with this idiocy.
Hopefully, Andy's site is logging the IPs of the posters as they post. Given the IP and the time of the post, he may be able to figure out which ISP the poster is using, and the ISP may be able to figure out which actual user is doing the posting and may be willing to take punitive action against him. Regardless, Andy can certainly delete the users on his site who are doing the offensive posting, and he might be able to prevent other userids from being created if the poster is from a particular IP. I suppose that all of this depends on the quality of Andy's board software, but he's not helpless.