How do I turn OFF Apple Pay on Watch but not on iPhone?

Change or remove the payment cards that you use with Apple Pay - Apple Support

How to remove a payment card on your Apple Watch

1. On your Apple Watch, tap the Wallet app.
2. Tap the card that you want to remove.
3. Scroll down and tap Remove.

OR

On your iPhone, in the Watch app, go to: My Watch > Wallet & Apple Pay > tap on a listed card > scroll to the bottom of the next screen and tap on Remove Card.

OR

If you turn off passcode for your watch and/or turn off Wrist Detection, all cards will be removed automatically from Apple Pay on your watch:

• On your iPhone, in the Watch app, go to: My Watch > Passcode > Turn Passcode off;
• On your iPhone, in the Watch app, go to: My Watch > General > Wrist Detection.

I hope this helps. :)