In the city you want to "disband": Build a worker or settler.
BUT! Be sure that the city is not growing! (Has no surplus food, or has starvation). Make citizens into entertainers to achieve this.
If the city is growing, the game will wait building the settler or worker until the city is large enough.
If it is not growing, you get a pop-up asking you if you want to disband it.
A worker removes 1 population, and a settler removes 2. I believe a settler takes more than 2 times more shields than a worker to produce, so workers are probably best.
I usually add the workers to another city, if I don't need a worker of course...
LeD