Loosen both of the screws that control the electromagnet. Do not take them all the way off. Pull the two wires off of the screws. This will disable the chime mechanism.
Why does my ADT doorbell keep chiming?
A low ADT battery constantly alarms you that the battery is almost empty, and the backup battery isn't fully recharged. ... In most cases, low battery notifications are exhibited through a beeping sound. For you to silence the beeping, go right immediately on the panel's keypad by pressing OFF or # buttons.