I think at that point I could have used a flat screwdriver and been good. Just put it on the solid parts of the plug and tap it firmly with the palm of my hand.
But my chisels are scarey sharp because I use them for guitar building and repair, and I thought a sharp chisel might help cut through. If you use one, just watch the corners and don't hit it too hard, as it will cut through the plastic around the knot hole or the bottom of the brush like a hot knife through butter.