It might not be a big enough hit to stop all circle grinders, but applying the script to only bosses might be a middle ground worth testing.
Or rather than making it so the boss gives no xp, making it so it doesn't even spawn. This would actually improve immersion(especially on the dungeons you need to walk back out of). You'd have to keep it tied to players rather than just having a timer to avoid disappointing other parties that come along shortly after though.
XP script suggestion: Hit the bosses
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