Terraria – simple way to identify the true Lunatic Cultist

Sprinkling vile powder or throwing unholy water will remove hallow. You can make vile powder from vile mushrooms at an alchemy station or purchase it from the dryad during a blood moon. Unholy water is crafted from a bottled water, an ebonsand block, and corrupt seeds. Note that unholy water will not just remove the hallow but replace it with corruption.

You want a timer or a crab engine (a statue-spawned crab trapped in a 1-high compartment with a pressure plate). Timers can be turned on and off just like a switch by right clicking them, so you can enable or disable your home security system whenever you like, or indeed you can use switches and other mechanisms to turn timers on/off.