Scenario - a user has come to the site multiple times over a period of 6 months. 72 logins over 10 different IP's.
We have identified them as a fraud, and have suspeneded their account. But they could always come back with a different email.
Now this isn't simply to prevent a random user from being an A*hole .. its actually that they used our service to solicit funds through our service (think Kickstarterish).
Any best practices, considering they probably will have a new IP next time they visit?