I figured I’d try out Diigo Groups by joining the one for “Roleplaying Games”, but most of the posts in there are from "WoW Gold Pig", which is (a) spamming for their own service and (b) not even targeting the correct readers, as the group is about tabletop RPGs, not computer RPGs.

I would recommend a flagging feature that allows group members to tag posts as spam or inappropriate, which would then (by default) hide them view unless someone explicitly clicked a "view flagged posts" button. The group's creator would automatically have a "moderator" status (and could then grant it to others) that would allow review (and reversal) of flagging; they would be able to deny flagging privileges to someone who was making inappropriate use of the function.

If the last moderator of a group leaves the group, moderation would then fall on the most active member, or most active non-banned flagger.