Unfortunately , I think you're 'up the creek without a paddle', as once marked with a sharpie the dye penetrates the spandex, you cannot remove the marker pen without solvent and that could /would damage the material , only cure is dye the spandex a darker colour than the marker colour , ths masking the mark. other wise start again and use 'drapers chalk' available from haberdashery shops or a dressmakers.