While not guaranteed, the best way to shrink spandex is to expose the garment to both hot water and extremely high heat. The only things needed to try to shrink spandex is access to a washer and dryer as well as a king-sized pillowcase.
The garment should be placed in a washer with water that tops 180 degrees Fahrenheit. After the garment has been washed, it should be placed in a king-sized pillowcase tied with a knot and placed in the dryer. The pillowcase will help to get all of the water out of the garment. The article of clothing should be dried on the highest temperature setting on the dryer to help promote shrinkage. This method should shrink a spandex garment one to two sizes, but most likely no more than that.
Spandex is typically comprised of above 90 percent of polyester, which is by nature a shrink-resistant material, as opposed to cotton. Cotton will naturally shrink and fade when exposed to high temperature, such as hot water and high heat in a dryer. An iron should never be used on spandex, however, as it will have the opposite effect. The fabric will most likely stretch and not shrink, in addition to becoming gummy and unusable.