Why not implement something along the line of asking the user to pick out a certain picture?

say for instance "pick out the square" and have a pick of 5 different shapes.
or "pick the object with 4 sides"

or ask "whats the word in 2 paragraph 3 sentence 5 word in the sentence" and grab some random text from somewhere

Basically implementing any kind of custom "captcha" turing test what ever yeh wanna call it
will avoid 99.9% of all the automated spamming due to the fact that someone will have to go out of there
way to figure it out and then create a program specifically for it to automate the spamming.