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 two marbles from two separate spray cans, 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.
Now it’s time for some crushing. Place the can on the ground and tilt it so the ball inside is out of the way at the top half of the can. Now drop something big and heavy on the bottom half of the can, like a brick or a sledgehammer. There is very little air still inside the can, so there’s no risk of any explosion.
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. 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.