And again, you missed my point. Banning by email address is not a wise idea since people may have multiple alias addresses, which means they can continue to create new accounts and thus use your forum.

Yea, okay, let's play devil's advocate...so, people have multiple alias addresses...so, you could ban by domain, which again would disenfranchise legitimate users on that domain.
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Buh Bye!

Cheers,
Me