One of my PunBB installations was spammed quite badly. We had to clean it up and lock it. Google read the forum topics .. so we just loaded it with good ones and denied new registrations.
Most were human, though. I noticed on the server logs that people found the website by doing a search for "Powered by PunBB", which I left in for copyright reasons. People used it to hunt down forum and spam it.
It's not a PunBB issue except to say that everyone who has forum should check the content at least once every day.