The "borders" in GCII are better termed "areas of influence". They do not have the same meaning as sovereign borders which no one can pass without permission.

In the parsecs and parsecs of empty space out there, how would sovereign borders be enforced? Post a fleet every 3-4 parsecs?

Of course, if someone crosses your area of influence without your concurrence, you have every right to blow him/her out of the ether! But that may have other, undesirable consequences...particularly if they are much much more powerful than you and could stomp you like a bug.

In a space-based game, this seems a reasonable way to do business and does not need "fixing" IMHO.

But shouldent there be an option to allow other races access to your space, so when your relations start going bad with the Dregin as usual they will have to declare war before crossing your border.

But shouldent there be an option to allow other races access to your space, so when your relations start going bad with the Dregin as usual they will have to declare war before crossing your border.

How would you know if they crossed your borders, most of the time the fog of war blocks you from seeing even if a ship is in your territory at all. the only way that you could tell is if a planet you know about but can't see is captured by some race. The impossibility of even being able to define sovereign borders is one of expenential impossiblity. you wouldn't be able to actually say that your border ends here and there's starts there. space is huge so large that borders mean nothing. the infulence is really to only differentiating detail.

Maybe not militarily, but it should be possibly diplomatically. While it is unfeasible to guard the borders of your space empire (even in game-time, if you are playing on a large galaxy), it should be possible to tell other races to stay away or you'll obliberate them 

I think that to please this reqeust we could have this: 2 types of border, one influence and one military.
The Military border is dependent upon how many ships you have and the total value of them. If you have no ships your military border won't expand far beyong the inhabited planets, but if you have 100 ships it will go right up to your influence border.
The thing is that if someone would enter that space it would usually mean they just entered the part they would seriously get their arse kicked in.