Turn off the circuit that controls the NuTone ringer transformer -- the main box that produces the chime sounds. Since you will be disconnecting wires, you need to be sure no electricity is flowing through them.
Pop off the plastic cover on the front of the transformer. It will come off with a little wiggling.
Unscrew the screw corresponding to the chime you wish to disconnect. They are labeled: side, rear, front. You need only turn it a couple times to loosen it.
Gently tug out the wires from underneath the loose screw. Tuck them to the side. Since the electricity comes from this end, you need not worry about live wires like you would if you disconnected the other (doorbell) end.
Put the cover back on the NuTone ringer. The chime is now disconnected. You can switch the circuit back on.