The problem of the game right now is not a player can't disband a city. As another poster mentioned, the new patch allows us one opportunity to disband it immediately after the city flips or is conquered.
The problem is, the game does not allow us to consult the detailed information of the city before we can make this irreversible decision. For example, one time I conquered a relatively small city, I destroy it, never realized the Pyramid was there (I thought it's in the enemy's capital). If I knew it, I would certainly have kept it since the early-game representation is already worth the increased maintainence.
I think the game designer should allow us to access the city screen before we make the choice. It's simply ridiculous that after you've conquered a city, and you still don't know what the haxk this city has before you are forced to order to kill off a city of a half million population with 5000 year of history.