This is a simple mini plugin, it shouldn't cause any lag.

Configuration:

Counter_Repeated_Profession_Change: true/false
When true a villager won't be able to lose a profession once it has one.
This will prevent players from repeatedly resetting a villager's profession to get a certain trade they like.

Disable_Villager_Iron_Golem_Spawning: true/false
When true villagers won't be able to spawn iron golems.
This will prevent players from making large villager based iron farms.

Feedback is always welcome.