To bring up Drinax again, it does make a point that while the biggest and best defense for true Imperial starports is the deterrent of "attacking a starport is a declaration of war against the Imperium", non-Imperial (especially Aslan) ports don't have that luxury. Generally Imperial starports seem to be armed enough to deal with any single ship or small flotilla, but assuming a true assault force they would rely on the Navy. It's specifically mentioned that on the other hand, Aslan starports are only considered to belong to whichever clan or corp built them, and therefore are armed based on that clan's level of paranoia/legitimate concerns, sometimes able to fend off a full invasion fleet.
I dunno how well that actually answers the question. In gameplay terms, I'd definitely say any A, and most Bs, would have little trouble protecting itself from a single non-capital-scale ship unless said ship were specifically built as a super-pricey death machine. The best option would really be to attack the shipping bound for it until the starport either gives in and negotiates or summons enough support to drive you off.
I dunno how well that actually answers the question. In gameplay terms, I'd definitely say any A, and most Bs, would have little trouble protecting itself from a single non-capital-scale ship unless said ship were specifically built as a super-pricey death machine. The best option would really be to attack the shipping bound for it until the starport either gives in and negotiates or summons enough support to drive you off.