If you do not have a service valve on your hot water supply to the hot tap how do you turn off the supply to change a washer?

Not much choice other than to shut off the water to the entire house at the meter or at a secondary valve that may be installed where the water main enters the house.

Yes, that is the only way, other than freezing the pipe using a special tool some plumbers carry, where if no valves are found or if all the valves are faulty the plumber will resort to freezing the pipes, this is a very complicated process and must be carried out by a qualified plumber, because you only have a certain amount of time before the pipes become unfrozen (climate depending) and if you are not finished by then, you'll be in big trouble.