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Separate names with a comma.
Ok, I figured it out with the ExplosionPrimeEvent.
The EntityExplodeEvent won't work for creeper explosions. I'm sure the event is registered.
Well, is there another event that get fired on creeper...
You could try it with /pex group [group] prefix [your prefix]
If it then not works, is the problem the chatmessanger.
Try to export the plugin with Java 7