If you have Find My iPhone enabled, you can use this to suspend or permanently remove the ability to pay with Apple Pay from the lost device OR
Go to your Apple ID account page, sign in and choose the lost device. Go to the Apple Pay section and click on either Remove or Remove all.
Even if your device is offline or not connected to WiFi or a cellular network your card(s) will be suspended or removed from Apple Pay.
Alternatively, you can raise a request with us.