In specific reference to the Junction, Manticore has the right (legally) to seal it tighter than a drum and refuse passage to all traffic. They do not do this for several reasons, not least among them being the high percentage of the bill for the war being paid by the Junction's use fees. There is also the fact--not yet sufficiently dwelt upon in the novels ('cause I can't find time to dwell on everything that needs dwelling on) that because such a good sized chunk of the trade along the periphery of the Solarian League passes through the Junction, shutting it down would impose major hardships on a lot of systems which would suddenly find their freight, passenger, and mail service severely disrupted. Then there are the political aspects of trying to close the Junction to diplomatic traffic on the part of neutral powers.
In general, a blockade in Honor's universe is much like a wet-navy blockade of the late 19th/early 20th centuries. To be legal, it must be effective--i.e., the blockader must possess the assets, deployed in the right places, to effectively spot and (hopefully) interdict incoming trade all around the system periphery. If he accomplishes this fact (and he can do it with LACs or even pinnaces if he likes), he is legally entitled to require any vessel entering the blockaded system to halt and submit to search and (where applicable) seizure and to open fire upon anyone, armed or unarmed, who disregards his instruction to halt.
In fact, of course, there has often been a very profitable traffic in blockade runners, but this is a particularly high-risk profession in an era of laser heads, and about to become even more so with the introduction of pod SDs and Ghost Rider missiles in general (not to mention the 2nd-generation LACs).David