I have hot water heating in my home. The boiler will not shut off once it is turned on. I tried replacing the wall thermostat but that did not work. The only way to turn it off is to hit the power supply to the boiler. Any suggestions?

The boiler is probably being controlled by the boiler operating control located on or near the boiler itself. The contactor or relay is probably stuck or burnt closed allowing the unit to continue to run. If the high limit is burnt/stuck closed this is not a good situation. If you do not know what you are doing I would call for ON SITE service to have a HVAC service person check out the system and make any necessary repairs.