Step 1: Release Pressure
I used the end of a socket extension. I bet there are pointed rocks that could also work in this situation...
Press down with your tool of choice onto the ball of the valve. Let all pressure escape, then do it again.
Remember that your keg may still have content. And it may be rancid. Point this exit blowhole in a safe direction at all times while doing this. That includes not standing over the keg while You do this. Lay it on its side for best results.
Step 2: Remove the Spring
Now that you know Where to begin hammering down, try again to release the pressure in the tank. It may surprise you to know that it takes a few times of doing this to release any and all gasses. Always point the keg hole in a direction without civilians or yourself. This could end badly for you or any innocent bystanders.
Step 3: Hammer Down
Step 4: Hammer Down in a Different Direction
Step 5: Remove the Spear
But since there is no more pressure in the tank, it can simple be lifted out once the tabs are aligned.
Step 6: Inspect the Keg Internals
My keg inspected clean.