How to cut off an invading force's naval supply?

[kalifriki1]

I've had the enemy land near a key city several times.
1) Is there any way to preemptively protect myself from the landing?
2) Is it possible to cut off their convoys without destroying the port (it's quite strong)?
3)Is it possible to somehow negotiate with MTH stop convoys (bombing their supply ships is impossible as i know)?

[Czert]

1. only by positioning units on coast (but not sure if there will be unit it will not triger amphibous landing and combat, i allways avoided opposed landing)
2. no onlyest way to prevent sending of supply is when two mth goes to war to each other and sink ships, so that mth suplying enemy will not have enough ships to carry enough supplies to enemy. yeah, sometimes it happen.
and i realy hated when i used shipyarding card to build ships for mth to have more ships, which will finaly have enough numbers to supply my forces effectively...only to regulary lose them in war against another mth. i realy wanted to shot thiers leadership.
so, destroying thiers (makeshift)port and ocuping that landing hex is onylest way how to effectively cut off enemy from supplies.
3. you can only negotiate with mth about yours existing contracts, no chance for enemy ones.