Are you sure it was "prep" instead of "perp"? A "perp-walk" is the slang name of a procedure that the police do with the "PERPetrator" of a crime. A number of criminals are handcuffed and they are escorted by police officers as they walk in a line to either a central booking location or to a bus which transports them to a jail. Sometimes the perpetrators wear leg shackles and/or arm shackles which are connected to the other perpetrators making it more difficult to run. The media knows when this will happen and they are waiting to take pictures. I believe your sentence means that the individual was handcuffed and "paraded" down the Main Street of the town so everyone would know what happened.