I used to run a pretty big video game forum. I also used phpbb . I see that you are running an older version, so to avoid the spam and the bots you should upgrade to the newest build which is phpBB 2.0.22 . At least thats the most recent stable release. After the new version is installed then you can install scripts that will prevent bots and spammers for the most part. Here is the site I got all of my security scripts from. http://www.phpbbhacks.com/category/10/
I know its time consuming to install a new build due to incompatibility issues with any add ons you may currently have installed, but its worth the effort. As long as you leave the mysql database alone the forum will retain all users and posts. I can lend a hand or even help with technical questions you might have . Doing this would make the site more secure.
What spammers and bots do is crawl search engines with keywords like "phpbb (version number)" and with those results the spammers and bots attack the forums using outdated software. Since this site is spidered by google and other search engines its easy for them to find the vulnerabilities in the site.
Hope this information helps. And If my post comes off as ripping your tech staff thats not it at all. I'm just a member offering some information that helped me with this problem when I ran my site.