Many stores have installed locking wheels on their shopping carts that are supposed to stop shopping cart theft. The cart gets a signal when it leaves the parking lot and one of the wheels locks up. This instructable will show you how to forge that signal and lock up anybodies shopping cart where you want (works inside and outside the store).
This will work on any cart that has the solid wheel like shown in the picture below. These are the wheels that walmart uses. This method works the same way as the EMP Shopping cart locker but is way easier to make, however the range is far less, about 2 to 5 feet.
I made this a couple weeks ago and I saw the high power one today and decided to post mine. It consists of an mp3 player playing the lock signal attached to a coil of magnet wire. When the mp3 player and the coil are placed in my shoe I can step about 6 inches from someones cart wheel and their wheel will lock.
Note: The picture is using the old coil, which had far less range, the coil I am now using is about 30 turns around a 1 foot diameter, with nothing in the middle. This increased the range to about 4 feet.
Step 1: The signal
I only have the lock signal, if you get the unlock signal you could probably play it the same way.