Recognizing people is a very hard thing to do.
I have implemented an unpublished
IP trapping mechanism that works like the VBulletin IP ban, except that anyone, registered or guest, coming in on an IP in that list will have a miserable session. This will not work with dial up users because they are coming in on an IP pool and I have in some cases 200+ people sharing the same dialup IP in my IP records.
But this works finr with users who have limited IP's. I have also added the IP addresses of some big name anonymizers to my miserable IP list.