So, I too had a problem with comment spammers which made me consider using captcha. However, the code wouldn’t validate and it was impossible to posts comments.
I found these two posts:
http://modxcms.com/forums/index.php?topic=23443.0
http://modxcms.com/forums/index.php/topic,24511.0.html
However, I’m a modx newbie and the solutions in there were too complicated or would take a lot of time.
My quick fix: use only one captcha word (Modx Manager > Configuration > Tools > User > Captcha words). Modx adds a few random numbers to every new call, so it still offers decent protection. With just one captcha word, the captcha now validates. (don’t ask me why, but it does