Sorry if this has been mentioned before, haven't seen it before though...
I'd like each nation to be able to set a specific rule regarding borders:
1) All nations you're not at war with, may enter your nation without asking (And use your infrastructure). If another nation decleares war on you, when having units inside your borders, this nation gets a reputation hit by all other civs.
2) A specific nation can get a permission to use your infrastructure (Like in Civ 3)
3) You can close your borders against a specific civ (If this civ crosses your borders, it's an act of war). You get a reputation hit by this nation
4) You closes the borders for all other nations (unless you've made a deal with a specific civ... as in point 2). If any nation crosses the borders, it's an act of war.
This could avoid the AI from keep crossing the borders, even though I keep telling it to "Stay out"