Fill a spray bottle full of vinegar and spray the inside of your microwave with it. Allow it to sit for three to five minutes.
Wipe the microwave out with a damp cloth to remove any grease residue and other stains. If the spark mark is only slight, the vinegar will clean it off.
Soak a cotton ball in nail polish remover and scrub spark marks inside the microwave with it. Soak as many cotton balls with the nail polish remover as needed to remove the entire stain. It may take five or six if the stain is bad.
Spray the microwave with vinegar again and wipe it out with a damp cloth.