Police can blockade a system. Navy blockading a system, however, could be viewed as Martial Law, and you could roleplay that out quite well.
All ships carrying an LPI ID are Liberty Police, and are allowed to preform all the normal functions the ID allows. In fact, you have more freedom as an independent than a factionised LPI.
Navy should not be at Jumpholes. That is metagaming. The only exception being Jumpholes to Guard systems.
It is against server rules to attack transports under any circumstances. The only exception, I believe, is if they are in your guard system and are not carrying the correct ID for that guard system.