It's bad behavior and against the rules to plunder your own Alta exclusively. That being said rule enforcement in this game...

Anyways what can he possibly do to you or the others besides whining? You can't plunder more than 3 times a day, even if he is in sage with you that means he controls several accounts and won't get rewards in his main one plus camping is not that easy.

Ignore him and keep plundering his accounts as per the rules