Babylon did the same thing to me. They had more territory than that, but they were the smallest Civ, and I was the largest. they also went ALLLLLLLL The way around my continent to the far side and dropped off the longbowman there. I mustered a coalition to keep him busy while I sent a bunch tanks over to take over a few token cities but I had no need to kill them off.
Someone mentioned that workers is better than settlers. It depends. You can only crank out one unit per turn, regardless of shield production, so if you already crank out 30 shields in a city, make a settler instead of a worker. The settler will build a city twice as fast with the same investment. Only make a worker if your shield production is less than 30.