I know it is far fetched, but when a server is listed publicly, you have a tendency to draw in the good and bad. Would it be possible to have a central database of "unwanted" leechers? A voluntary subscription for servers that are listed publicly.
Another option, is two lists, one that is "open to the public" and another that is "communities". We would like to list our server for people to find and join, but we aren't a open to public server, if that makes sense.