There isn't one office in charge of that. All taxi companies have their own call center to take customer's complaints.

If you use an official taxi company, with the fapiao you got (providing you got one), it contains all the information to give to the call center to track back the taxi driver (taxi fare, distance travelled, etc). They will check your complaint with the taxi driver, and if fund "guilty", the driver will be punished by the taxi company and you may get a partial or total refund of your fare.

If you use a "fake" taxi, then you can just report it's plate number to the police station and issue a complaint there. However, if the problem is unsignificant, the police station won't even register your complaint.

If you have a problem with a taxi driver, do not give up the funds, pull your cell phone out and dial 110, show it to the driver, see how fast his mood will change. Never push or give a Chinese citizen an attitude, this never goes in your favor, if you are polite and have the police help this will not be a problem. By the way, never ever ever, have a taxi drop you off anywhere other than a public place, or this can get you hurt. Other than that coffahoilc, chalk this one up on the the board and forget about it.