When you are permanently barred, you have to wait outside the United States for ten years before you can even ask to come back into the United States. There is no possible waiver during this time, and the waiting period does not even start until you have left the country.
Even after the ten years have expired, you have to ask for special permission to return to the country. The government has a lot of discretion to grant or deny the request. To get approved, you would have to demonstrate some especially compelling circumstances.