I bought a USA iTunes gift card from a Thailand store,but the card have been used already and the company said they aren't the ones that used it

You cannot redeem a USA card with an account set to Thailand, but it appears pretty clear you were scammed and bought a card that had already been used. If the store owner won't refund your money then you have learned an expensive lesson to only purchase cards for your country and to only buy them from places you can trust.

This link will take you to the section of the support article about what to do if your card has already been redeemed If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support