Inject a rule into the background simulation - any time these players form a group bigger than "X" and remain static in a system longer than "Y" spawn NPC's of increasing number and strength until the group moves on.
Why bother. The game is already gathering data that adds a bounty to your avatar. What is to say it isn't gathering other stats that indicate the type of player you are. The game mechanics can then dump all players of certain types into their own instances. They may not even have to be that canny. If player A has player B on their friends list, but twenty other players have player B on their ignore list then the chances are those twenty other players wouldn't want to play with player A either.
Groups like the Mittens will always travel in packs, it's necessary to their style of dominance. That will make them vulnerable to instance isolation. So what if they control the Sol system, and brag about it. All we have to do is send our commands into our parallel Sol, have a meet up, a party and post the videos on YouTube.