OK, first-off, the LAPD is NOT a trunked system. They are P25 conventional & you would need a radio that capable of the R2 UHF split for 450MHz-512MHz.
Second, as a conventional system, he could program the channels to be TX-inhibited by choosing the 'Receive Only' option in the personality settings for that channel (you would get an error tone if you touched the PTT).
Please try and keep the facts straight, even if you are trying to talk the OP out of doing what he wants to do...