A good natural way of killing off Crabgrass is to spray the Crabgrass with straight vinegar without diluting it.
Vinegar will kill off Crabgrass and then you can spray the lawn with Crabgrass prevention chemicals that keep the Crabgrass from growing back.
Remove any dead Crabgrass plants that you may still have in your lawn to prevent any Crabgrass seeds from spreading further into your lawn.
Pouring boiling water onto the Crabgrass can kill off the Crabgrass and boiling water can kill off other weeds too.