The Taxi and Limousine Commission. These people (in NY anyway, presumably in other states) issue the taxi drivers licenses. These people have teeth to discipline. They are totally separate from the cab company & are not the overburdened police. They exist to regulate the cab industry. This is their job. And look, they do follow up: http://www.wnyc.org/articles/wnyc-news/2010/jul/19/cabbies-surrender-licenses/
This is their website: http://www.nyc.gov/html/tlc/html/home/home.shtml . It looks specific to NYC, but it may help you find the licensing agency in your area. There has to be one, even if it's called something else. *Someone* has issued those drivers permits to operate taxis.
Here's something else to think about: many cab companies are no longer structured the way they used to, ie there often is not one single entity that owns the fleet of cabs, pays the drivers a certain amount, pays expenses and tries to make a profit. An increasingly common arrangement is that each cab driver owns their own car (or even several cars) and acts as a private contractor underneath the umbrella of "Black and White Taxi," or what have you. There can be levels of subcontracting beyond this as well. I discovered that when a cab driver offered me a flat fare one day when they don't usually do that. I thought the price was fair and went with it, but when I questioned him, he gave me this whole big explanation. Basically, each cabbie has freedom to operate his/her business the way they feel right. They pay a certain fee to the umbrella company for the right to use their name and have their dispatcher book calls for them. Beyond that, it's theirs to operate under free market practices. So, there may not even be a management to complain to.