When members go rogue and other community frustrations

I have a rogue group of members in my community. They don't think I know it. But I know it all too well. Their tactics are completely juvenile. They plot against other members, and pat each other on the back when their antics cause others grief.

They clearly have a lot of time on their hands. One of the members was once an employee and I believe this person is trying to settle a score by bringing what my grandfather would call "ill-will" to the community, which in this persons mind  causes...