I believe I know the reason for spam on my recent post 'Forum' my conclusion is that someone has set up software to read the security captchas, they are far too easy to identify with third party software and you should work on a secure captcha background, instead of a plain white background put some triangles or squares on the design so that the text is still easy to read, but software that reads it won't work as it won't know whether the square is the captcha, or whether it is the background, this SHOULD help protect against bots.
P.S. you might want to work on blocking certain links with the 'a href' tag as you can hide bit.ly links etc by linking photos/text. That or notify people that clicking links on a thread could be a potential risk as they could download viruses or steal information. So if any users that read this post click links on a forum, please remember on mobile devices/tablets you can hold a link and see what its actual URL is, and on PCs, right click and inspect element.
I hope this helps get rid of bots posting spammy adverts and potentially harmful links on posts. Thank you for taking the time to read this.
kieran.
The suggestion is great and now it's on the consideration!