>>16
Honeypot fields is a good start.
Even just two checkboxes where one is hidden for the user with css tricks that don't make it too obvious it's a honeypot, and another checkbox that literally just says "check this if you are not a bot". A bot will see both of them and likely fuck up clicking the wrong one or both or neither. A user will see one and click it and submit.
Probably one hidden field that pretends to be the real comment field as well.

That will filter out 99% of untargeted spam.
Until you get targeted spam, nothing more complicated needed.

Most spam that comes on imageboards and textboards I believe to be done manually though. I have witnessed so many boards get the same spam, and many of them had custom written captcha solutions and got the same spam next day.