How to turn off Apple Pay on iPhone SE 2nd generation

You can prevent access to the Wallet app which would include using it for Apple Pay purchases by going to Settings > Screen Time > Content & Privacy Restrictions > Allowed Apps > Wallet.

If you are trying to accomplish something else, then you will need to describe what that is. For example you can prevent in-app purchases in the App Store or disable the App Store so no purchases can be made there. Other options include removing cards from the Wallet app or turn off Apple Cash in Settings, all of which prevent any Apple Pay transactions.