I don't know if there is a way to disable keyboard and mouse, but you could create an event structure and add event cases for keyboard and mouse functions using the filter events, Key Down? and Mouse Down?. These are found under <This VI> event selection. For both cases, wire a True constant to the Discard terminal on the right side of the event structure. That way, any key pressed or mouse click will be discarded. You may have to also add the Key Repeat? event.
In order to enable again, instead of wiring the True constant, you would have to create a True/False condition (like a Disable button), and wire the output of this condition into the event structure to be wired to each Discard terminal. Remember, a True condition will Disable the keyboard and mouse.
Just how are you sending the keys to Notepad? Sendkeys will cause the keycodes of the keys to be sent, just as if you pressed them on the keyboard. "abcd" keycodes are hex 61, 62, 63, 64. Perhaps this is showing up in Notepad as 1,2,3,4. Whatever function you send the keys to must be able to take the keycodes and translate into characters.