By the usual law system. Police find the suspects, send it to the prosecutor, the court, lawyer, judge and all.
You're missing the part of getting evidence. Seems downright impossible to prove.
I believe it will go along the lines of a sting. Get enough complaints, send enough undercover cops on said bad rides (short routes / racial differences - whatever the drivers are being accused of) and get em.
I was going to say there's no way there's police budget for this kind of enforcement in the US but I would've been wrong: