we have a new spam attack - and new unleagal "subscriber" users

Hi
For 4 days we have a big spam attack and a lot of "subscribers". we don't have any registration form in the site, just the people who writes a question or an answer - and they have a different role - SDK users
also our customers who write an answer getting a lot of spam and some start to leave us because of that.

we have "stop spammers" plugin up to date and running, but still we have this issue.

any help?