I have two shops with electrical service each fed from a separate breaker at the box next to the meter. I am going to tear down one of the shops and need to abandon the service to it. How do I deal with the breaker for that shop?
First off, since you're working in your service equipment, I'd have the power company cut the power to your place for the duration if I were in your shoes (thanks to smart metering, many electric utilities can even do this remotely -- call them and ask). You'll also need a pair of BRFP filler plates to blank out the opening in the box the breaker will no longer be occupying -- you can order these in at your local electrical supply house, and something (an appropriately-sized screw lug connector or four is ideal, although a thick wrap of electrical tape will do the job in a pinch) for insulating the ends of the feeder wires so they don't contact something else live in the box.
Once you have all the materials and the power cut -- remove the deadfront from the panel, then unscrew the terminals (two on the breaker, one on the neutral bar, and possibly one on the ground bar as well depending on the age of the feeder and how it is bonded) for the feeder wires in question and pull the wires out of the terminals, insulating their ends as you go. Install the feeder plates into the deadfront openings corresponding to the removed breaker then reattach the deadfront. Finally, have the power company turn the power back on.
If you don't want to fiddle with all this, of course, you could simply turn the breaker off, lock it in the "off" position to keep some knave from turning it on behind your back mid-demolition, and mark it clearly as "unused".
Don't walk, but RUN to your favorite licensed electrician.
Taking on a project like that can be life-threatening.
For instance, if you have a neutral )or ground) fault along the way, you could end up with what you think are neutral wires that have voltage on them.
Not worth your life to "try" - instead, go to someone who knows.