You didn't say what the aircraft / engine are, but in my experience a fixed-pitch prop won't stop in flight even if you switch off the mags, or pull the mixture to ICO.
My disclaimer is that there may be some engine / prop combinations where you can easily stop the prop in flight. However, in my experience you have to slow the aircraft to just above the stall to stop the prop in flight after switching off the mags.
Further disclaimer.... (Don't try this at home). Although you have difficulty in stopping the prop in flight, if it does stop you will also have difficulty restarting it, i.e. to get over the first compression stroke. It may require a steep dive up to VNE, and even then it may remain stationary, and you will probably have used up 2000 feet of altitude.
If you ever have to do this for real there are two tricks left: pull at max g out of the dive, or roll the aircraft opposite to the prop rotation direction. Remember, however, that you will be way over design manoeuvring speed.