Put the wok on the stove. Heat it over high heat until it begins to smoke and water drops sizzle when you splash the pan. Turn the heat off, but keep the wok on the burner.
Pour 1/2 cup salt into the bottom of the wok. Add enough cooking oil to form a paste when you mix the two together with the cloth. Be very careful not to touch the wok with your bare skin.
Scrub the rust stain with the salt and oil paste. The salt will turn brown as it removes the rust. Continue scrubbing until the stain is gone.
Rinse the wok in clean water. Heat it over the stove again to evaporate any remaining water. Allow it to cool, and store or use immediately.