I don't ever recalling of actually having the ability to attack escorts under your control, somehow that ghost escort is not supposed to be a bug. I believe that was intentional so you don't accidently kill it. Now that doesn't mean that you can't destroy it indirectly, if you have a forklift, you can destroy a target if your escort is nearby, or other weapon that can do a superficial blast damage. These weapons can work.
And by the way, if these are hired escorts, couldn't you just hail them and press the release button? I mean that's a sure way of ridding escorts if you don't want to pay them anymore. In my situation, I prefer captured escorts, and if I can't capture any successfully, I just simply hire them, release them and attack them to capture them. I get a better chance of capturing than finding a particular ship I want.
------------------
Nosumus Fortiolis Quad Volimus