Mail.ru, ukr.net and Yandex.com email accounts banned

You should see the stats over here: http://stopforumspam.com/

1.7m spammer accounts get checked every time someone registers, and because new accounts are created by the spammers every time a forum admin detects one they register it on that site and then it's blocked from all forums.

How big does the database of spammers have to get for this to be impractical due to lag of iterating through all the known spammers? Or is this not a problem until the DB is huge?