I'm trying to spawn murderers and scarecrows at a specific time and despawn them at a specific time I used halloween.enabled true and murdererpopulation 100 and that worked to spawn them but I tried halloween.enabled false and murdererpopulation 0 but that didn't despawn them so is there a way to kill them all off at a specific time ? I tried nightzombie plugin but that doesn't work for me.

The EnforceLimits methods in SpawnHandler might do the trick.  Set the population to 0 and try that.