here's a few plugins I would suggest. We had this problem on our forum with the little bastards making new accounts with bogus email addresses, using ip-proxy servers to hide behind, and so on.
This combination of plugins has pretty much locked the little bastards out.
Multiple Account Prevention & Detection
https://vborg.vbsupport.ru/showthread.php?t=199077
Verify Mail Before Registration (verifies they have an actual working email address)
https://vborg.vbsupport.ru/showthread.php?t=201466
Proxy to Real Ip Conversion
https://vborg.vbsupport.ru/showthread.php?t=201211
Multiple Account Registration Prevention
https://vborg.vbsupport.ru/showthread.php?t=127045
Limit Account Registration by Domain
https://vborg.vbsupport.ru/showthread.php?t=188418
we also shutdown new account registrations that come from yahoo, hotmail, gmail, msn, live, and other free email webservers. so if they use email from a fee provider, they can't register. our forum requires them to have an email address from their actual Internet Service Provider (comcast.net, swbell.net, pacbell.net, bellsouth.net, att.net, att.com, verizon.com, t-mobile.com, etc).
hope that helps man, cause I know it's frustrating.