[Puke]

i dont know if you could make it impassible, but you could create a destructive border between subsystems. wormholes would have to be placed in each system with the map editor.

also, you will have to tweak crystiline solar resource generation. lowering it by 1/4 or 1/9 will actually be harder on crystiline races, as there will be no trinary systems for them to take advantage of.

and stardestroyers will be really harsh in this... ouch.