Can't turn off security light permenantly
It sounds as if the light is being controlled by a single 3-way switch. The hot lead is run to the common and two switched hots are run to the traveller terminals.

You could create a master switch by adding a single pole switch that interrupts the hot lead before it goes to the 3-way. You could expand the box holding the switch to a 2-gang type and wire the new switch with the existing hot lead and a short pigtail from its second terminal to the 3-way common.

You also could do the same with a double switch in the existing single gang box. Either a single pole/3-way combination or even a 3-way/3-way combination would work.



In this case, the pigtail runs from one terminal of the master switch (the single pole switch) to the common terminal of the 3-way. if you are using a dual 3-way, just leave one of the traveler terminals unused.

In either case, the white (neutral) and ground leads are left connected, just adding a pigtail from the grounds to the new switch.