What's the best way to restrict an environment to a specific CDN?
If the different CDNs are hosted at different IP addresses one possibility might be to configure the local hosts table for the machines in a certain environment in such way as to overwrite the DNS resolution for the CDNs which shouldn't be accessible and pointing them to either:


a non-existent address, causing connection timeout failures if the invalid CDNs are referenced
a server where the respective files cannot be found, causing 404 errors (faster than the connection timeouts)