Attempting to force a stuck nut off a threaded bolt with a wrench can sometimes cause the nut edges to become rounded. At this point, a wrench becomes useless. Nuts usually become stuck due to rust or sediment build-up between the nut and bolt threads. The best way to loosen rounded-off nuts is with a pair of curved jaw vice grips that can get a tight lock around the nut.
Brush the exposed threads around and above the nut with a stiff wire brush. This removes some of the rust and sediment build-up that prevents the nut from turning.
Spray penetrating lubricant around the seam between the inside of the nut and the threads. Allow the penetrating lubricant to soften the rust and sediment for 30 minutes.
Lock the jaws of a pair of curved locking pliers tightly around the rounded nut. Straight jaw locking pliers do not hold securely enough to rounded bolt heads and nuts.
Hold the bolt head or screw head with either a socket wrench or screwdriver, if possible. Ideally, ask a friend to hold the bolt or screw head while you loosen the nut.
Tap the top of the locking pliers handles with a mallet to turn them in a counterclockwise direction. The tapping usually forces the threads of the nut past the initial sediment and rust build-up.
Retighten the pliers around the rounded nut and attempt to turn the bolt or screw head while holding the vice grips steady. Sometime it is less difficult to remove the bolt or screw from the nut.
If the nut still will not loosen, consider cutting the bolt between the nut and the mounting surface.