Report, and don't back down!
I had an incident with a city employee who was driving an SUV on a MUP. I asked him for his ID and he very briefly held it up, then drove off. Initially his superiors started telling me what wonderful things this fellow was doing as part of the city's outreach program to the homeless.
My response was that he was driving a motor vehicle on a MUP with no purpose other than being too lazy to walk and refusing to allow a citizen to verify his ID. Eventually, we reached an agreement that he should not drive on the MUP unless there was a need to carry a lot of stuff or an emergency situation. Also, that when asked to present ID he MUST allow citizens to take a picture of it with their (now ubiquitous) cell phone camera.
I haven't seen him on the MUP again.