Bribing

[Sand]

Assassins are the answer - kill every enemy diplomat you see. All of them. No survivors. High influence diplomats are a good second choice to protect your cities or stacks, but killing diplomats solves the problem permantly.

If assassins are failing to do the trick....there is the dark art of surrounding an enemy diplomat with a ring of units [8, or less if you can call some impassable terrain into play]. When youre sure the diplomat cannot move [check the yellow boundary], then move another unit onto the diplomats square.

Yeah I know, but as they say - the AI gets the cash and you get the brains.