I think you're trying to delete the ACE (Access Control Entry) and I don't think you can delete those nodes because it relies on inheritance mechanism where ACL permissions given on user node are passed down below '<user_node>/rep:policy'
Even if you try to delete a ACE node from CRX as admin and save it, you will see a message like this
Could not save changes. Received 409 (Conflict) for saving changes in workspace crx.default. Cannot remove protected node: no suitable handler configured.