I recently bought a washing machine which only uses one inlet hose. The one that isn't been used now leaks even though I have turned it off. It is beginning to rot the kitchen cupboard and I fear it may rot the floor boards.
Can anyone help?

It could just have crud on the seat of the valve. Put a container under it and let the water run a little, and then close it. If that doesn't stop it, shut the water off before it where ever you have to. Note, if you shut the water off to the inlet of the hot water tank, you first must shut off the electric or turn the gas to pilot. You don't have to do that if there is an expansion tank between the shut off and tank. Any time the tank is heated, water must be free flow back out the inlet as it expands. Current practice has a back flow preventer with an expansion tank between it and the tank.

Then remove the nut below the handle and remove take whole works out. Check what you take out and the seat inside. If the seat is good, and the stem had a bad washer on it, just replace the washer. If the seat is bad, you will need to replace the whole thing or cap it of.

If you are uncomfortable with that, you can buy a cap in the garden hose area, and screw it on. If you need more details, post back.