For that part go to Edit Common Receive SW and then turn sustain Off

Having already created a multi voice performance, how do I switch on/off the SUSTAIN PEDAL function on a voice without affecting the rest of the voices.

There are no "voices" in the Montage. You are referring to Parts. You can activate or deactivate controllers on a per Part basis.
Press [EDIT]
Press the PART SELECT button [1/1]-[8/8] that corresponds to the one you want to edit 1-8
In the lower left corner of the screen select "Common" (blue)
Select "Mod/Control" > "Receive Switch"
Find Sustain and deactivate it
Press [STORE]

If you continue to call them "Voices" (sic) you will never be able to refer to the manual or the front panel and learn your Montage. You're editing a "Part".