A simple plugin that will kick you if you damage a player or a players things if the purge is not ongoing.  During the purge the alarm sound effect goes off and the bright red text "THE PURGE HAS COMENCED". Only when this text is displayed are you allowed to destroy basses and kill players. This would be such a simple plugin and tons of fun too. 

There would need to be a countdown timer displayed too and possibly a randome min and max for the overall time of the purge and time till next purge. etc etc

To confirm.. you're looking for an admin controlled window of time (called the Purge) where pvp is allowed, but outside of that window it is not allowed?