I have found a way to safely remove the ball from inside an empty spray paint can. This is a tried and tested method that I used to retrieve the marbles from the empty spray cans I’ve found, so it does work. Be advised I’m posting the details of this method for informational and educational purposes only. Try it at your own risk.
First and foremost, make sure you’re doing this outside! The first step is getting all the air out of the can, so start by holding down the nozzle until the hissing stops. It should be a few minutes before all the air is out of the can. There might still be a little paint inside so be prepared for some splatters of paint. It’s a good idea to use the inside of your garbage can to minimize the messiness.
Now it’s time for some crushing. Place the can on the ground, after making sure you’re wearing shoes, stomp on it. It should dent easily with the very little air remaining in the can, but if it’s still firm it might not be safe to proceed as there’s still air and possibly paint in the can. A clogged nozzle is usually the culprit here.
This is the part where it gets messy and a little sharp, so some gloves are a good idea right about now. Fold the flattened portion of the can against itself and then quickly fold it back in the opposite direction. Repeat this rapid back and forth folding pattern and a hole should begin to form along the fold. You may hear a slight hissing or see a small amount of paint leak out, so be sure to do this over a garbage can. Soon the hole should be large enough for you to use a screwdriver to pry it open and get to the marble inside. Use a paper towel to wipe off the excess paint before enjoying your free marble.