Playing a game at the moment with a relatively strong Hannibal as a neighbour reminded me of a game some time ago where he was a neighbour and, in combination with others, I can conspired to severely limit him. Through various wars (which he had been provoked into declaring) he had been whittled down to three cities and was no threat to anyone. Approaching the late game I had quite a reserve of espionage points against him that I decided to use. This is the point at which I was despicable....
Carthage was stuck in the middle ages whilst the rest of the world was industrial (and I was tech leading in the late modern period), so there really was zero military threat. His military resources consisted of one iron mine. But I didn't target that with my spies. Instead, and just for fun, I went in repeatedly and took out every happy resource he had, and then took them out again when he rebuilt them. This, for me, was truly low. I had no real reason to keep his people in misery; I gained nothing by doing it. It was just for kicks....
EDIT - IIRC I giggled like a borderline psycho as I did it.