Yes - I wired the bell wire of the crossing signal to a switch on my control panel (it's the red one) so that I could stop the bell from ringing if it got too annoying. I don't remember which wires I used, but it should be apparent from the Z-stuff instruction manual.
If there are separate wires going to the speaker, an on/off switch on the hot wire between the power supply and the speaker should do the trick.
I did this with my MTH crossing with bell, good thing too, its almost as loud as areal one LOL!