Depends on the amount of traffic you're expecting. If it's very light, rolling any kind of captcha of your own will work.

For example, on a moderately popular (~100 comments a day) phpbb forum I used to run, I had a custom captcha that was a single image and a single correct answer. It completely stopped all my spam issues.

Eventually I would have needed to upgrade it, but it never got to that point. Nobody wanted to specifically spam my forum.