We recently had a customer that wanted to use Cloud Flare as their Content Delivery Network, DNS provider. Although they wanted ALL traffic to be routed through Cloud Flare and any traffic not going thru cloud flare should be declined. You can do this thru access restrictions in Microsoft Azure.
Login to https://portal.azure.com
On the left Select App Services
Find and choose your app service for your business
Once you have choosen your app service you will want to then click Networking
Then find Access Restrictions and click Configure Access Restrictions
You will then select Add Rule then type in the Allow IP addresses and Deny all Rules.