If you ever get into a situation where your site has over 50k users or so, you will see banned users switch email addresses to circumvent your security measures several times a day. I know I do, and my site only has 300k users.

The solution is to store banned IP's in your database, excluding massively-shared IP's like AOL, and check against this list during registration. I also keep a list of free proxies and prohibit people using those IP's from signing up.