Apply post-emergence crabgrass herbicides. These herbicides kill the crabgrass and will completely eradicate any crabgrass populations when used in conjunction with a pre-emergency herbicide. Example herbicides include Fenoxaprop and Dithiopyr, and target crabgrass without harming lawn grass species.tep 3. Use pre-emergence herbicides. These chemicals prevent crabgrass seeds from sprouting, breaking its life cycle. They do not affect already-growing plants, thereby protecting your lawn.