How can you deal with a severely autistic child's fits when they become violent?

Depends. It depends on what you mean by violent. If by violent you mean life or limb threatening then you will need to use medication (oral or injectable) to terminate the event. You can also use similar meds to prevent the fits. Teaching self soothing skills and hypnosis can be of value to prevent the fits of rage as well.
Combined approach. best dealt with as a combined approach with cognitive behavioral therapy, provided by a behavioral psychologist and medication. amantadine is particularly effective although other medications have been used. speak to your pediatrician for referrals.
Autism. Many children with autism have more agressive behavior. Keeping them on right schedule on daily basis helps avoid any unpredictable changes in behavior in response to change in schedule. Medication that helps with mood disturbances might help too.