Which TWO wires to disconnect to disable USB mouse
My first reflex was to say that you should disconnect D+ and D- lines. Leaving VCC and GND connected is obviously safe for both the mouse and the host: VCC and GND leads in USB connectors are even made longer to insure this safe state is reached before D+ and D- are connected.

However, disconnecting VCC alone may be enough, and even if not, it should at least be safe. Take a look at this figure from USB specification:



The host will only communicate with the device when it sees D- line pulled up. However, once VCC is disconnected, Rpu is effectively eliminated from the circuit, leaving both D+ and D- at ground potential. As a result, no further communication will take place.

Once you reconnect the VCC line, the host will again see D- line pulled high, and will react as if a new USB device was just plugged in. In the best case, your mouse will just work. In the unlikely worst case scenario, the mouse won't have enough time to initialize (since normally VCC is connected before D- and the device has more time to start) and will fail to reply to the host within the allotted time. In that case, you'll have to keep VCC and GND connected and put a DPST switch on D+ and D-.