Making a defensive perimeter around the fief/castle/town or whatever would certainly eliminate poaching/smuggling. I find that to be a great negative, as it's best to support independents, neutrals or smaller factions being able to do stuff. However, I believe having a system similar to that of recently total war games, in which armies have zones of control in which another army cannot pass. Of course, the troop limit to impose a large zone of control would have to be quite large. You could prevent S and D smuggling if you sit a nice 800 man army near the place, sure. But that's a large use of resources that is likely more useful elsewhere. There would surely be unguarded fiefs to smuggle/poach from. Of course, for this to work, I believe there would have to be an integrated diplomacy UI in strategus. That'd be a bitch to code and create, I'm sure.