Just thought I would share this with you as over the last couple of months I have noticed the odd dodgy bot registered user on this forum and I'm guessing there are many more we don't see.
I used to get this a lot on the forum on my site http://www.windsurf.me.uk/forumpage.html
until I installed a little bit of php that gets rid of the character based authorisation as you have now (which is very easy for the bots to read) and replaced it with a very simple and fun visual system. It is easy to install to php-fusion too and should take less than half an hour.
I have stuck with the default pics but it is very easy to change the images and questions so instead of the cute fluffy things I have you could replace it with questions like click on the starboard logos or click on the iSonics.
Just try and register on my site and you will see how it works, you don't have to complete the registration, just go to the registration page.
It has stopped bot based registrations completely and would work really well for the guest users on the free forum