How to Make Your Own Shopping Cart Locker

Pretty clever stuff from the website Instructables if you have the know how!

The "EMP Shopping Cart Locker", here's their disclaimer:

"Disclaimer: This is not an easy project. It will require knowledge of circuits, soldering, social engineering, and a tiny bit of PIC microcontroller stuff. High power is involved, and if you mess up, you might get burned, caught on fire, or arrested-- most likely all three. Always use an appropriate amperage fuse to prevent a short from becoming a much bigger problem. This project is also fairly expensive. The parts will cost about 65 bucks. Batteries with some kick will run 20-30 bucks, and a single PCB is about 60 bucks. They get a lot cheaper if you buy multiples for your friends."

The "Shopping Cart Locker THE EASY WAY (Improved Range)" is a no frills method and can work in a store and outside/anywhere! Or, why not use the "James Bond Style Cart Locker" These methods and the one above require the carts to already have the stopping device on the wheels.
Or, you can try the "Portable Shopping Cart Locking Force Field of Doom". Here you can make a force field around you to protect yourself from carts.

I don't think you should put the law in your own hands, but it's definitely entertaining to read!