The point of origin for that wire has to be inside one of the boxes fed by the GFCI circuit. You should be able to find and disconnect it from inside the box. If you don't want to go to the trouble of figuring that out then just put an electrical box on the end of the wire and nail it up on the floor joist. Cut the wires off if exposed and put a blank cover plate on the box. Also staple the wire up. You could also use the new box as a junction box and continue with new wire to somewhere else where you might want a new outlet. Was there a sump pump are anything like that under there? Kevin