Remove Garbage Disposal romex or cap?
Your best and safest path forward would be to purchase an old work box and a blank cover. Instal the old work box in the wall with the wiring inside the box. 
Nailing a box to the cabinet would violate code in many locations as the romex must be protected under the sink.

Old work boxes are inexpensive and made for jobs like this. With the wires safely capped inside the covered box is the least expensive way to do this and the circuit can be used in the future if needed.