Are you using civ3[vanila]? In some earlier version there is no "abandon citt option". To destroy you must keep lowering the population (by starvation or settlers/workers) unitl it gets to 2/1. When at two if you want build a settler. When at one build a settler/worker. Eventually it will almost be finished. It will give you the option to demolish
WAIT: I think it might be this one instead. If you have a metroplis starve it down to a city (must be 12 population), then sell your hospital. Then wait a little while. When the popup appears click abandon city (Free setler :happy

. If it isn't a metroplis, well, you get the point.