I am happy to help you out with this today! To prevent a check from being cashed, we will want to void the check payment within QuickBooks Online. Voiding a check will place a Stop Payment on the check.
A few things to note:
A check with a Stop Payment on it will be rejected when presented at the bank.
As a courtesy to the vendor, please inform them that the check is no longer valid. This is in order to prevent any fees from their bank that might incur if they attempt to cash it.
Checks can be voided until they have been cashed by your vendor (and cleared by the vendor's bank).