how about banning them through email addresses?
are they all comming from the same place? isn't there a way you could ban anyone from registering if their email address is @something.blah
if not, it cant hurt to ban all the individual email addresses seperately
also, check the IP addresses, if they're all similar, but their endings are different, you can ban them all easily, for example,
say 2 of them use these ips:
123.45.37.88
123.45.37.42
you could just ban 123.45.37.*
or even 123.45.*.* if the last and 2nd to last digits are different.
if they're completely different, it still wouldn't hurt to just ban each one individually, im sure after a while you would've banned them all.
....just trying to help