I don't know if this has been reported yet, but currently you cannot destroy NPC Capship Turtets. I've experienced this with the Unrat captains (sorry don't have a time frame), but I've been told it's the same with all NPC Capships.(Deneb, Convoys, Blockades).

Edit: I should clarify, you cannot even damage them.

We should have this fixed on the next server restart. Thanks for the report.