I have a great example for you. I just made my siren a remote mount and learned this much about it.
Whelen 295HFSA1. Its a 200Watt siren (200W air horn is great!)
The 295HFSA1 has a Power rocker, a tone knob, a PA volume knob, an air horn button and a manual button. Open the case (4 screws on the front) slide the inside out. Find the back of the manual button, and cut ONE wire to it (both wires are black and unmarked, so cutting both would require marking before cutting, so you can restore its function later if you can use it or if you resell)
After that, put the siren back together. Set the tone selector knob to P A. Remove the two small Allen set screws that hold the knob on, and remove the knob. There is no way it will turn without the knob or pliers! What you are left with, is an Air-horn button, and a PA.
The tones aren't "disabled" but you cant turn them on, and this way is REVERSIBLE so if laws change, departments change, or if you decide to resell, you can go back to a full feature siren with a couple Allen screws and a butt splice or soldering iron.
Here is a pic of the siren (from radioreference)
http://forums.radioreference.com/attachments/old-classifieds-archives/23536d1247181084-federal-pa300-whelen-295hfsa1-siren-0714209-030.jpg