Have you contacted RTD about this?
http://www.rtd-denver.com/ContactUs.shtml
Get the bus number, route number, time of day and location. Whatever description of the driver you can come up with will help too, even if it's just male or female. By bus number I mean the number painted on the bus that RTD uses to keep track of buses for parking, maintenance, etc. The route number is on the destination sign.
The police can't do much unless they see this happening, but RTD management will probably take a strong interest in your story. RTD buses have video cameras, I'm not sure if they can be used in a customer complaint.