Apparently this is not a firm rule. The Enterprise is very kill-able in Klingon Academy in the Academy Final Exam Mission. I have delighted in destroying the Enterprise over and over again. The Enterprise is scripted to warp in and around Earth during the Klingon invasion. If you concentrate all fire on her warp engines, the kill is easy enough. This may be a technicality. The Final Exam is actually a simulation programmed by Admiral Chang, so the kill only exists in Chang's private fantasies.