Okay, you can block by IP but that will only result in any IP block and nothing else. So, how do you do this across the board permanently? Can't be done. How can you do this across the board to make it more effective?
First, you need a program that tracks IP by member in your admin panel thus keeping track of the IP's and the e-mail addresses.
Next, you need to isolate out the e-mail to keep it from being an issue in the future. So, here's your path:
One, block the IP
Two, don't delete the account, rather suspend and ban the account from your Admin Panel, but do not delete it. This will keep the members e-mail account in your database and at least remove that e-mail from being used again.
Now, those who are smart enough will simply obtain a new IP (Not an impossible task, but it can be a pain in the arse with the advent of DSL/Broadband internet now) and a new e-mail. Experience has shown after you block them a few times they will eventually go away as it's to much work for them to keep joining. Also note: If you block an IP in the Admin Panel, make sure to add that IP to your sever's list of blocked IP to do away with little idiots who try to backdoor your site in a temper tantrum.