Capping Off Wires and Covering With Blank Plate?

I have Jacuzzi tub on a 20amp circuit with a GFIC on it, that has a bathroom add-on outlet tied in to the circuit with 14 gage wire. I feel sure this is a code violation. I can not get to the junction box to replace the wire with out tearing our the tile around the tub. What should I do with this outlet? I was thinking of capping off the wires and putting a closed plate over it. Would that be safe?