Add Cooldown for Event triggering

Quick question can players spawn the npcs using mine HQM, kill them all and gain tons of RP, then turn the excavator off, then switch it back on again to create an infinite loop of NPC killing? If so can you make it so their is some sort of cooldown on the NPCs spawning? Otherwise players will exploit this and endlessly spawn npc waves on my 100x server to get insane RP amounts. Thanks so much for any help this is a very cool plugin! Other plugins do too much, I just wanted defensive npcs wihout all the other things devs add so this plugin is perfect for our server.