How do you stop payment on a certified check under misrepresentation?

Contact your bank. Unfortunately, the answer is probably that you can't. UCC 3-411 makes banks who refuse payment on a certified check liable to penalties. There are some defenses against that, so it might be worth contacting the bank anyway, but don't get your hopes up. You're probably going to have to let the recipient cash the check and then attempt to sue them to recover damages.