Fist this is bad.. You never mess with an electric blanket it can cause a fire or kill him.. Besides if it is "broken" he may request a new one. Like noted buy a timer for it make sure it is rated for the power the blanket pulls..
Buy a timer and put the plug into it.
http://www.screwfix.com/p/masterplug-plu...
So I'm not sure if disable is the right word, but step-son keeps leaving his electric blanket on and I want to make it appear that it is broken. Obviously I could just take the blanket off his bed, but I can't be bothered to deal with the tantrum that will ensue. We don't live in a cold house and we have central heating, and it's June! Fed up with him leaving the blanket on all the time, and looking forward to saving some money on the electric bill. Thanks.