by Ken Golliher:
Write a check for the account balance to purchase a cashiers check payable to the owner. Hand the cashiers check to the owner and say, "I closed your account."
by John Burnett:
Ken has provided a concise and easy solution, and the only one I support, since there may be reasons that the authorized signer wants a clean and legally supported "break" from the account. One caution, though. If the owner of the closed account brings the cashier's check back to the bank and demands that the account be reopened, you should require that a new account be opened. You need a new deposit contract without any trace of the old authorized signer in the paperwork.