If the drivers are in any way even possibly breaking the law, then the riders could go to the police. If the drivers were acting in a completely lawful, but undesirable, way then the riders could give the drivers a low rating. It is very safe to assume that the servers of Uber (and all other ride sharing services) work to opaquely disconnect passengers and drivers when either party has given the other a low rating. It's a good and obvious idea on all levels, including even just optimizing for max profit.