What’s really missing here isn’t a forcefield in space, it’s a diplomatic button that says “quit it, you’re pissing me off”.
I will echo this comment.
Right now, we do have the means to enforce our territory. I routinely will attack AI's that are trying to send a constructor or colony ship towards a spot that I want or that is in my borders.
The problem is that war is my only option....which is the unrealistic part. History is full of people who both made claims to the same land. But they didn't always resort to war. There was intimidation, payouts, espionage sure....but not every border conflict becomes an open war. That's what the game is missing.
Now as far as whether influence is a "real border" or not...I don't mind which way they go put I don't want it half way. Right now, the open borders option says to me "influence is a political boundary". If it is, so be it. If its not, drop the open border option.