Tom, I disagree, the longevity of old content and the ability to easily re-start the conversation even years later is one of the positive features of the OpenACS forums. Good discussion lasts. The gazillion PHP-based forums out there should try to emulate
it, not the other way around.
All the OpenACS forums need is a small tweak to make the posting date more prominent; the current hard-to-read grey date color is unwise. Perhaps also distinguish very old posts with some sort of "n years old" message next to the date.
Abuse by spammers is a separate and entirely orthogonal topic.