The old way was to put them in a tea chest sized box with a slatted floor. Keep it clear of the ground so there is a gap under it. They can't settle to sit then.
We tend to make sure they have no eggs to sit on and every time we pass the hen house, lift them off the nest and put them outside.
Of course, a sure fire way to stop a broody is to go out and buy some really expensive setting eggs and put them under her, she'll stop sitting next day!
Dave