How can I remotely unpair and wipe an Apple Watch?

Go to www.icloud.com and login with your Apple Account credentials. Click on Find My, then select your Watch and click ‘erase’. The remote erase also unpairs the watch. If you want to gift it or sell it then you also need remove it from your Find My list of devices.This assumes your Watch was previously setup and added to your Find My list of devices. If not then you are out of luck as far as remotely erasing it.

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support