Would it add anything to be able to warn, turn, or otherwise escort ships away from your territory rather than fighting them? Say that you're not at war with a player but for whatever reason (probably trade related) you don't want his ships in your territory. Turning a ship away asserts your control of your territory without the escalating effects that would come with destroying or capturing the ship.
Intriguing.
Do you mean, an actual forcefield or tow-lock of some kind? Or a political sanction? I feel that a mere chat system would allow for these types of warnings, would they not? Perhaps I am misunderstanding your point here though.
I'm thinking of is where you have two players who are not at war but not really allies and one decides to try and probe a bit into the other player's borders. Or perhaps a player wants to implement some sort of blockade. A chat system alone could work but if you have two players who aren't officially at war and they have a fair number of ships to watch over it might be something worth automating somewhat.