If a Notice of Deficiency has been sent, it's already too late.  IRS has a form for agreeing that it be rescinded, but no one has ever seen that done.  Clients should be told that they should not have let the case progress to that point (although sometimes, of course, it was not their fault) and they should be prepared to pay the $60 Tax Court filing fee.  81% of Tax Court cases are "pro per" -- I think many of the petitions must involve help from professionals not enrolled to practice there.  Do your own research on what state law allows.