By SPAMmers i mean bots made to flood forums by crappy ads.

1. change custom antispam question (AdminCP)
2. consider implementing phpbb module that is using and go to AdminCP/spambot countermeasures and set reCaptcha Google's recaptcha or keycaptcha
3. See in which forum sections they SPAM the most (probably its only couple of them), go to Moderator panel and then back to these spammed forum sections and mark all posts and uncheck only not SPAMmy posts. Hit delete to bulk delete posts.
4. use Search function to search for posts containing common words/footprints of SPAMmy posts to bulk delete them
5. Use AdminCP to delete users without posts. (AdminCP/Users and Groups/Prune users; posts equal to 0)

to further protect phpbb from SPAM one can add second laer of protection, custom antispam question. If recaptcha, keycaptcha was used in AdminCP spambot countermeasures, then one can manually add questions in AdminCP/Users and Groups/Custom profile fields, here it is described. (takes around 20 minutes of time)