Restrict Google from indexing subdomain (Azure CDN services)

How can I stop Google from indexing content, we have the website from last two years but just recently we find out Google has started to index all the URLs on the azure CDN subdomain website, and most of them has already started ranking. We have been using this subdomain CDN for all our assets. So not sure what made Google to start indexing all the main URLs all of a sudden.

I have amended robots.txt file, and has blocked Google from crawling it but somehow it is still crawling them. If someone know any other way to direct Google straight to custom domain and if anything needed to be changed at Azure dashboard.