The easiest thing they can do to stop merc exp parties and alliance or such, would be to implement what they use in Campaign battles: The less work you do to help kill the targets in a campaign battle gives you less reward points, experience points, and rewards from coffers, etc.... basically the less you do the less you get.
Implement this ability in ANY party set-up and it would kill exp merc parties that charge for it. Because they players would not be able to sit around afk doing nothing.
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Just a thought....