Most likely, that was a result of security holes in the version of PHPBB forum (3.0.6) we have been using.
Today I have upgraded the forum to the latest release (3.0.

I hope this will scare away bad robots from our friendly community.
The forum was out of reach due to upgrade process for around 30 minutes today.
Sorry, my fault, I didn't bother to let you know beforehand.
Pasha