What you think is a dark, dirty stain around your neck might actually be a medical condition. One of the symptoms of acanthosis nigricans is a thick, brown ring around the neck that is often confused with dirt. Before you take drastic measures to try to remove this dark ring, see a doctor to be certain it's not an undiagnosed condition. Once you get a clean bill of health, you can proceed to work on getting your neck clean.
Scrub your neck with a nail brush while showering. Use a strong, powerful soap such as lye soap. Rub some soap on the brush and scrub the neck with it, then rinse well under the shower.
Exfoliate with an exfoliating scrub product that has little beads in it. This should remove dead, dry, dulling skin cells. There are numerous exfoliating products available at most grocery stores and chemists.
Try whitening the skin, if scrubbing or exfoliating doesn't work. The ring might be dark skin, not dirty skin. Prepare a mixture of the juice of one tomato with one lime. Apply the mixture and leave it on for 15 minutes. Rinse off with a washcloth and cool water.
Use a commercially prepared skin whitening product if the natural skin whitening method doesn't work. These products contain a variety of ingredients and chemicals to whiten skin.