Cut the black wire connected to the motor. This vibration motor is soldered directly to the main board, and there is no way to disconnect it without severing the cable.

As an alternative to fully disabling the motor by disconnecting it, you can take a metal file to the counterweight on the motor. Removing excess mass from this weight will drastically soften the vibration

Using a small bit of electrical tape or glue, cover the ends of the wire you snipped so they don't short out on anything else inside the device.