Batboard

Is there a way to safely and completely disable the transmitter on a spectra other than leaving the mic unplugged and still have it receive and scan ok?

I have one that was setup to receive only on a 800 meg smart zone system (setup legitimately by my local FD) and I just want to make certain that it can't transmit ever.

For monitoring trunking you can inhibit the transmit in the personality options. F4>F4>F3>F9. There is a section there that says "Transmitt Inhibit". This defaults to "Disabled". Change that to "Enabled" to disable transmit on a trunked system personality. To add to your safety, you can always use an invalid ID number and incorrect connect (PL) tone. Just make sure you have "Auto Affiliate" turned off in the personality.
As far as conventional goes - There isn't a programming way to inhibit transmit. You can always program the freq to a simplex channel, set it up for low power, and use a bad PL. Other than that it will still transmit.
One final way to inhibit transmit altogether is to make the ignition sense to allow PTT, then never hook power to the ignition sense.
963Expert

to disable tx on a conventional channel, type in "BLANK" for the tx freq

The safest way I found is to relace the PA deck with a deck from a 900 spectra.
I usually buy 900 spectras for parts.
I have done this on a few and tested it. Works , no problemo.