How to block the phone

Follow the steps in the support document you posted from to Mark it as lost:

Mark as Lost

When you mark your device as lost, you remotely lock it with a passcode, keeping your information secure. This also disables Apple Pay on the missing device. And you can display a custom message with your contact information on the missing device. 

Mark your device as lost.

Again: Mark it as Lost. This locks it and prevents anyone from using it.