I thought my forum, being relatively low-traffic, and using the relatively obscure punbb, would stay spam-free for a while. No offense to punbb, but a quick google shows only some 624 sites with punbb having 12 or more threads (search punbb inurl:viewtopic inurl:id=12). Hardly a big target for spammers, I would say.
Too bad, after six months, I got a spamflood: 43 posts with links to medicine websites, each from a different ip address, all unregistered users. And two days later, again 43 posts. I put in a home-brewn captcha for anonymous posts and new user registrations, which successfully blocked another 158 messages over three spamfloods. All this within about 2 weeks.
So to answer the topic question: yes, I suppose spam is a problem.
I have a question about the claim that captchas hinder accessibility, and that that can become a legal problem. I put in a message that anyone having problems reading the captcha can contact me by email. Wouldn't that be enough?
Is there a place where I can report offending IP addresses?
Update 26 June: the counter is now 610 successfully blocked spam posts, in one month after installing the Captcha.