Be finished with, as in I hope we've seen the back of Betsy; she is terribly rude. This idiom transfers literally seeing someone's back because they are leaving to a more figurative and permanent departure. Also see see the last of.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.