There are a few options:

Send them a letter saying you realized that their return is too complicated for you.  Refer them to the tax preparer in your area that you absolutely despise.  Kinda like killing two birds with one stone.

Bill them $100,000 for the return.  When they complain, tell them you will reduce the fee down to the same amount as last year, but you won’t be able to reduce it next year.

Find a new office and move there in December.  Don’t tell the troublemakers where you moved to.  As you are leaving, put a note on the door saying you retired.