@EventHandler
public void onEntityTransform(EntityTransformEvent e) {
  if(e.getTransformReason() == EntityTransformEvent.TransformReason.CURED) {
    e.setCancelled(true);
    Bukkit.getLogger().info("Event was cancelled!"); //Enjoy flooded console
    //I was spawning a new Villager here. Almost killed my server with whole horde :O
    //Currently, developers need to manually kill zombie here to prevent this event
  }
}

if (CraftEventFactory.callEntityTransformEvent(this, entityvillager, EntityTransformEvent.TransformReason.CURED).isCancelled()) {
    this.getDataWatcher().set(EntityZombieVillager.CONVERTING, false);
    this.removeEffect(MobEffects.INCREASE_DAMAGE, EntityPotionEffectEvent.Cause.CONVERSION);
    return;
}