Please halls do something about it, in my forum i had the same problem, but it's fixed thanks to this phpBB add-on:
http://www.phpbb.com/customise/db/mod/nucaptcha_plugin/
or check more here: http://www.phpbb.com/customise/db/modif ... ti-spam-9/
Also, go to the member management and ban those bot's IP because they only use one.
If you need help just say, but im sure you don't need
![Razz :P](./images/smilies/icon_razz.gif)