There seems to be a new type of griefing that has become popular which is to loot parties. It appears to be the same small group of people doing this over and over and over to any party that one of them can get an invite to. Basically every time one looter is booted the looter's friends invite them back so you can't get rid of them all.
The solution is very simple. Make it so what you boot somebody from a party they can't get back in. Make it a permanent removal.