If you lose your Apple Watch, you can put it in lost mode.
Go to the Apple Watch app on your iPhone.
Tap My Watch, then tap All Watches.
Tap account.apple.com using your Apple Account and remove your cards.
In the Devices area, choose the device.
Click Remove Items below Wallet & Apple Pay, then click Remove Items again to confirm.
You can also call the issuers of your cards. For more information, see the Apple Support article What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else.