It sounds like the hot water pop off is jammed in the open position. It may be made to close, or it can just be replaced. It should unthread from the heater. Take it with you to the Hardware store and get an EXACT match. Take it home and reinstall. DO NOT JUST PLUG THE HOLE. This can lead to the water heater building enough pressure to EXPLODE and kill you or any else nearby.
Should be in the 20 dollar range.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato