LOK-A-BOLT “Door Angel”, is a simple device that is placed over a “deadbolt knob” when the deadbolt is in the locked position, thus preventing the deadbolt from rotating enough to retract the deadbolt. This assures that the door cannot be unlocked from the outside. This amazing device is simple and inexpensive, yet delivers exceptional protection against intruders who may have keys.
http://www.lokabolt.com/howdoesitwork.htm <--This in the website that it can be purchased from, for $15 plus shipping. The website explains how it works but I am a little worried about the sexist tone on the website. The big banner says "Women love it!" What about wimpy men! Lets get with the times Lok-A-Bolt! Also in their description it says "protection against intruders who may have keys, such as landlords, building superintendents, ex-husbands, ex-boyfriends, thieves, sex offenders, etc." What about ex-wives and crazy girlfriends? But other than the lack of equal opportunity descriptions, it looks to be a good product.