Now I could be completely wrong on this but I seem to remember it mentioned somewhere that only a solar system might be regarded as belonging to some power but the space inbetween always is uncontested, kind of international waters where everybody has a right to be.
I would imagine it also being very impractical trying to really control space other than systems just because of the sheer size of it. There are exceptions like the neutral zone obviously.
On the other hand, while space is really big, there are only so many ways you can travel to a system, sooner or later you are bound to arrive at it, so setting up listening post/monitoring devices at the edge of said system shouldn't be that hard to do, so I imagine it isn't that much of problem.
Anyone played ST: TNG A final unity? I think they did a pretty good job on the star-map there. No definity borders or such nonsense in space, just a cube filled with stars which might belong to this or that faction. A nightmare to navigate but pretty realistic imho.
I'm Commander Shepard and this is my favorite store on the Citadel.