Just disconnect at the breaker? Or, remove the old wire completely?
I can see that one might wish the wire to remain in place; there will likely be another sign out there someday.
Too many places operate their signs at the breaker, though, for me to be comfortable leaving the wires hooked up. Often the panels have markings by the breakers saying something like "On at night" or "Off at Closing." It's too easy for the remodelling crew to just flip everything on when they want light.
