When can I legally stop payment on a check?
Check written on 03/0709 to an appliance store but not cashed. Can I legally cancel the check now?
Your best approach to this might be to simply do nothing.
The first thing I'd suggest you do, is go to your bank and talk to an officer about this check. It's more than six months old, now, which many banks regard as "stale dated" and refuse to pay, pursuant to section 4-404 of the Uniform Commercial Code (a statute that has been adopted in nearly-identical form by almost every state). However, that law does allow the bank to pay the check. If your bank might still pay the check, you could put a stop-payment order on it.
You should understand that if you do stop payment, or the bank refuses to pay the check, the store can still sue you for the money, if you received merchandise for it.