I had a couple of ideas on how to slow these fools down...

1. When an IP address creates more than one account, place an incremental wait timer on the next account creation. (0m,20m,40m,80m,160m,...)
2. When you blacklist a character, make it account wide.
3. While were at it...if I blacklist one character on someones account, I do not want to hear from any of that accounts characters.
4. Allow your players to assist you in the war against these annoyances - give us an option other than blacklist that we can use to report them - no command line, just message response option - after an account receives enough of these reports, you can have your way with them.
5. Options for the user: Allow tells only from Friends/Free Company, Block friend requests

Just a few suggestions that might give you some ideas