Here's the Tutorial on What to do if your Apple Watch with Apple Pay is lost or stolen?
Get help if your Apple Watch is lost or stolen!
To remove the ability to make payments using credit and debit cards on your device, go to iCloud.com. Sign in, click Settings, choose your device, and click Remove All. You can also call your bank to suspend or remove your cards from Apple Pay.
*If you turn off Wrist Detect, remove your passcode, or sign out of iCloud, all credit and debit cards will be removed from your Apple Watch.