This ible walks you through the process of easily removing the valve from a Sankey keg.

Step 1: Release Pressure

The keg even if empty will still have pressure. This will end badly for anything in range of that spear as it flies out at a pressurized rate.
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

Where the spring cool ends, is where the real work begins. Start with some sharp pointed metal object, inserting it into the crack where the spring ends and the neck of the keg meet.

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

Now begin tapping that spring out. Using the tool as a lever to get one full ring out, then it's easy to do the rest by hand.

Step 4: Hammer Down in a Different Direction

Counter clockwise, tap the tab with your metal pointed object and hammer until the tabs align with the holes in the neck of the keg.

Step 5: Remove the Spear

If your into theatrics this could look like pulling the sword from the stone.

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

If there is calcium looking buildup on the walls or spear, thoroughly clean the thing with hot soapy water and a scrub brush if possible.

My keg inspected clean.