Most flashlights can be safely stored just by loosening the tailcap about a quarter turn.

Some lights have a lockout mode (commonly four clicks of the switch, if it's a side switch/e-switch), which can be handy. Lights running the Anduril 2 (not sure about Anduril 1) firmware by ToyKeeper can even set an automatic lockout timer. But if you want to be 100% certain it's not going to turn on, the easiest thing to do is just loosen the tailcap a bit, which will break the circuit between the battery and the pcb.