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Please remove the cover and then may remove one of the wires and carefully tape off with electrical tape or cap off with a wire nut to prevent the bell from ringing, see picture below of the terminal screws on top of the unit >
reference > http://www.nutone.com/common/productDigitalAssethandler.ashx?id=5c6ee893-f1bf-400f-a3eb-8435f55a27b7
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Does it matter which of the wires to remove?
No it does not matter which wire you remove as once remove anyone the circuit will be open and no longer energize the bell. . . The middle terminal will be the easier to remove as is the 16vac low voltage AC common terminal, while the other outside wire terminals are for a front double chime and rear single chime connected for the push buttons. So if remove the 16vac center common will disable both outside front and rear button from operating, otherwise if only have two wires, removing the common or front will remove the power to the bell.
or if do not want to mess with the wires, can take a piece of cardboard and place it between the metal strike pins of the bell and the metal plate >