I've had to reduce the permissions a few times and have done this to get it done. The most recent example is that I've had people spam the shoutbox. Not actual spammers, just people posting bizarre things on there. So I noticed that everyone that had done it recently had less than 5 posts.
To initially create the list I go into member management and have it select all users with more than, or less than, the number of posts and then change groups and add them to the new group. It take a minute or 2 to do it but it is the easiest way that I know to do it.
