Danieyl,
UCC Sec. 4-403 provides for oral stop payments and that these will lapse in 14 days if not confirmed in writing. The statute does not cover the situation where a customer denies having requested a stop pay made orally because it is presumed that the bank will confirm that it comes from the account owner or authorized signer before it is put in place. Several banks I know have modified their stop payment provisions to remove the requirement of confirmation in writing and simply place it on the system for 6 months. Other banks have modified stop payment provisions to make the stop pay permanent. As indicated by John and Randy, you must accept the oral stop payment, and I am sure your account disclosures indicate how long the stop pay order is effective.
Now whether you process the oral stop payment immediately is another matter. The Oklahoma Code Comment:
1. The right to stop payment is subject to four limitations: (1) It can be exercised only by the "customer" or other person authorized to draw on the account and, as to the stop payment order itself, it must (2) identify the item with reasonable certainty, and (3) be received "at such time" and (4) "in such manner" as to be effectuated before final payment. "Customer" is defined in subsection 4-104(a)(5). Any one of multiple account holders may stop payment or close the account, even if more than one signature is required to draw on the account. But see UCC ยง 4-405.
Sec. 4-303 permits the bank to establish and disclose a cut off time:
a) Any knowledge, notice, or stop-payment order received by, legal process served upon, or setoff exercised by a payor bank comes too late to terminate, suspend, or modify the bank's right or duty to pay an item or to charge its customer's account for the item if the knowledge, notice, stop-payment order, or legal process is received or served and a reasonable time for the bank to act thereon expires or the setoff is exercised after the earliest of the following:
(1) The bank accepts or certifies the item;
(2) The bank pays the item in cash;
(3) The bank settles for the item without having a right to revoke the settlement under statute, clearing-house rule, or agreement;
(4) The bank becomes accountable for the amount of the item under Section 12A-4-302 of this title dealing with the payor bank's responsibility for late return of items; or
(5) With respect to checks, a cutoff hour no earlier than one (1) hour after the opening of the next banking day after the banking day on which the bank received the check and no later than the close of that next banking day or, if no cutoff hour is fixed, the close of the next banking day after the banking day on which the bank received the check.
(b) Subject to subsection (a) of this section, items may be accepted, paid, certified, or charged to the indicated account of its customer in any order.
If no cut off hour is established, then case law indicates it must be placed within a reasonable time period. Depending on the resources and procedures of the bank, that could be anywhere from 5 minutes to a couple of hours.