Forum Spammed

While it has been pretty quiet around here lately (busy busy busy), it does not stop the FuCoder forum getting spammed. I am having not-so-fun time deleting hundreds of posts over the last two days (while my home Internet connection is down). Actually I didn't click on "Delete" on each of them but a single SQL does the trick, but it is annonying nevertheless.

It turns out PunBB has pretty much non-existing spam protection. Neither are plugins of any use. The forums were opened for guest posting so people asking questions do not need to create an account, but it has been abused by the spammers (or spambots?) to flood their junks on my website.

So for now, no more guest posting on the forum -- you need to register a user. It has certainly slow down the littering rate, but I know it also introduced inconvenience to those who just wish to ask one question...

Any other good light-weight open source forum software that has good spam protection?