Willow and Willy chewed our cords up when they were kittens. We purchased Critter Cord http://www.crittercord.com/critter_cord.html
It's a long, plastic hose with a slit in it. You put your cords inside it. The plastic is infused with a citrus taste. It did the trick. They didn't like the taste and quite chewing. We were able to remove it after a few months as they no longer were interested in chewing.