[Parasite]

Police cars pull people over all the time. They mostly don't have anti-vehicle weapons on board. The cars (or invisible freighters) know that BAD things will happen if they don't stop. I put "Don't get hurt" orders on my blockade ships. This way I know that they will not get killed by the imobile planet defenses when they "Strayed" too close.

I like the idea of leaky blockades. Part of it could be based on the planet defenses. A small unarmed and remote colony world would not leak much for fear of being blasted. A heavily defended ringworld could surely sneek out stuff and be glad to do it to spite the blockaders. Enemy and friendly ships in the system could also change things.