You may be able to patent a new use for an old thing, but you can't patent the thing itself unless it's somehow different from the old thing. What you may be able to do is patent the new method of use. For your example, you might be successful in claiming:
A method of preventing a door from closing, comprising:
inserting a shoe horn between a bottom of the door and a floor, thereby wedging the door in a fixed position.
(This is a crappy claim, and it may be invalid as a single-step method, but hopefully it communicates the idea.)
A method-of-use claim is often inferior to an article-of-manufacture claim, because you would have to sue each end user who buys a shoe horn and uses it to wedge open a door...you'd probably prefer to sue the manufacturer of shoe horns, or go to a retailer and seize all the shoe horns off their shelves. But if you really just have a new use for an old product, you probably don't have much choice.
An important part of designing patent claims is to consider, "who will make this device?" or "who will do the steps in this method?" That's the person who might infringe the patent, it's the person whom you might be able to sue, and from whom you might be able to collect damages or royalties. If it's an individual end user, it's likely that the cost of suing them will exceed the money you could recover from them, so that sort of claim may not be worth the cost of prosecuting it.