When you cannot shut off the hot water in the tap, the source of the problem is either a broken water control handle or worn-out parts in the tap's valve stem or cartridge. Damage to the delicate teeth or splines in the back side of the hot water handle affects how they line up with the grooves on the valve stem or cartridge, making it impossible to turn the parts in the stem or cartridge to shut off the flow of water. Worn-out parts in the tap also cause water to flow freely.
Turn off the water to the bath either by twisting the handles in the access panel on the wall behind the bath's plumbing clockwise or by closing the main water valve in the house. You will know the water has been shut off when the hot water stops coming out of the spout.
Remove the set screw in the hot water handle, which is located either at the base of the handle or underneath a small plastic cap in the middle of the handle. Once you have removed the set screw, pull the handle off the tap and examine the back side of the handle for signs of damage to the teeth on the handle. If the handle is damaged, replace it with a new one.
Use a spanner to twist the valve stem or cartridge out of the tap assembly counterclockwise to remove it. If the tap uses a cartridge, simply install a new cartridge by sliding it into the tap assembly, lining up the notch in the cartridge with the notch in the water pipe.
Undo the screw at the end of the valve stem, if the tap has a valve stem, and unscrew the packing nut. Remove the rubber washers as well, then coat the new washers and packing nut with plumber's grease and install them on the stem in the same orientation as the old pieces. Screw the stem into the water pipe clockwise, using a spanner.
Slide the hot water control handle back onto the tap and replace the set screw as well as the plastic cap, if there is one. Open the water valve in the house.