How to get rid of claimed cities

If it's your city, you can abandon it.

If someone else claimed a city in your region & never came back, unfortunately, no. There's no way to force an abandonment or institute a reset on another mayor's city. It will be marked abandoned after a year (real-time) of inactivity from the mayor. 

The only real way to avoid it is to make a private region and only allow cities to be claimed by friends. If they stop playing the city, you're still in the same boat, but at least you can ask them to come back and abandon it.