I don't think there's anything do be done. As already stated:
1. Users can get tons of disposable e-mails for new accounts
2. Proxies exist for registering new accounts and avoiding the last-login-from check which could be used in some cases
3. Rigs can be rented so they are assigned to one account only

Steps 1-3 could be performed countless number of times, and there's nothing we can do to avoid that. Hey as they said Internet provides countless possibilities