Have you read this?
https://vborg.vbsupport.ru/showthrea...70#post2146070 (and maybe tried some of the suggestions?)
I'm not an expert on the subject, but I think first you'd want to do what you can to try to stop as many new spammer registrations as you can. Then I'm not sure what you can do to stop spammers with existing account except to ban them when you find them.