In my experience, aircraft circuit breakers are not designed for use as switches - they are there to remove power from the circuit in the event of a fault condition that causes that circuit to draw excess current. I know that it is common practice to 'pull' breakers when the equipment on that circuit is known to be faulty or to disable things such as landing gear horns, etc.
Routinely using breakers which are possibly only certified for 1 - 200 cycles as switches will result in the breaker wearing out prematurely and requiring replacement.
If this is a bit of equipment that you regularly wish to isolate, fit a proper switch. If it's a 'one off', then I'm afraid that with a flush breaker, I don't believe there's a way to do it and you're looking at getting a licensed engineer to disconnect the circuit and properly insulate any floating wires.