When a poltergeist is destroyed, it only remains destroyed for 2d4 days. After this time, the undead spirit reforms where it was destroyed, fully healed. The only way to permanently destroy a poltergeist is to determine the reason for its existence and set right whatever prevents it from resting in peace. The exact means varies with each spirit and may require a good deal of research, and should be created specifically for each different poltergeist or group of poltergeists by the GM.