Guess we'll need some anti-bot modules for phpBB. Did anyone use such a thing? On the main website all of them are a bit outdate..
Update1: Installed ATLBL, but now I have to wait for an API key if they send them out. Hope they do and the plugin still works. We may have to move to different forum engine if the spammers won't stop.
Update2: Got the API key, they replied really fast (thanks again!). Hope this will solve our problem.