How do signal jammers block police radar signals?

From your question, there seems to be some confusion. Radar detectors are what people put in their cars to detect police radar. Radar detectors are equipment installed by police or police in areas where radar detectors are outlawed, check your local laws.
Signal jammers are different from radar detectors. While detectors are basically just receivers, jammers are radio frequency transmitters. Jammers, an active countermeasure used by the military, emit powerful radio frequency signals in the radar band to obscure the radar signature of police radars. Radar jammers are illegal across the country, not only by local authorities, but also by the FCC. You must be authorized to use such equipment.