Make the distinction between what's mechanically possible and what's legal. If you put on something you refer to as a stop payment in order to make the item reject on presentment, that's fine. If you actually send a cashiers check back "payment stopped" you bought yourself a law suit you will not win.
If its drawn on your bank and signed by your employee, then it's a cashiers check. As such, your bank "accepted" it at issuance and you cannot change your mind.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.