Neil (
with one noted exception - see below), the dirty little secret about using CAPTCHA verification codes is that they are easily defeated by spam-bot automation. Your forum is clearly a popular one, so you're going to be a particularly attractive destination for spam-bots.
But here's the good news: Spam-bots can be stopped cold with either
Carsten's SpamFruit mod or
Derek's Anti-Spam Question mod (
there are YaBB forums that use both).
Note: The BoardMod software is needed to install these mods (
downloaded from this page - although as I understand it, manual installation can be implemented).
The
BoardMod Project's website is here.
Also, and I apologize for not having a link to it here, but somewhere there's a video that shows you how to use the BoardMod program. Maybe you can ask about it over there (
you'll need to register). Seeing that video was a great help for me.
In terms of sorting out code, I'm actually in the same boat as you are because a lot of coding is way over my head. In fact, sadly for me, I had to do a lot of reading about how to install, and use BoardMod before I could use it. I was at the bottom of that learning curve.
Thankfully, I got encouragement from people here in this forum to give BoardMod a try. Gratefully, I also got invaluable help and information directly from Derek about the
Anti-Spam Question mod he created.
Note: I think the anti-spam mods may have been written specifically for YaBB 2.5 - but they both work perfectly fine with the 2.4 version I use. I see that you use the 2.2 version of YaBB, but I'm pretty sure other admins have been able to successfully install these mods with that 2.2 version.
Recommendation: Read about these mods on the
BoardMod Project's website, and study how they are to be installed.
Alternatively, in what I now call
"Plan B," you may want to do what I did when this stuff was just too much over my head, and implement a CAPTCHA modification that for our forum stopped spam-bots cold.
Affectionately and gratefully, those of us who have used it call it
"ggn's anti-spam hack" (
current details about how to implement this clever manual modification can be read here).

Thank you again ggn!

Good luck to you and your friends who are using YaBB!