I've stuck mine in a top loading washing machine before. It doesn't really ruin the rope, however be ready to spend an hour untangling the mess afterwards. You can get away with washing it in a machine with an agitator if you put it in a pillow case or bag. Front loading machines are the same way. You should run the machine through a rinse cycle before you wash your rope so you can be sure there is no residual detergent in the cylinder. Nikwax makes a good rope wash, I might suggest using an extra rinse cycle also.
But like Erik says, handwashing in the tub is a great way to do it too.