I needed to find a way to restrict ALL Azure Service Manager (ASM, aka Classic) resources on the subscription level. Azure Resource Policy seems to be a logical choice. So I quickly developed a very simple Policy Definition: View the code on Gist. Once I have deployed the definition and assigned it to the subscription level (using PowerShell commands listed below), I could no longer deploy ASM resources:
#Set the Subscription ID
$subscriptionId = '7c6bd10f-ab0d-4a8b-9c32-548589e1142b'
Select-AzureRmSubscription -Subscription $subscriptionId
$definition = New-AzureRmPolicyDefinition -Name "restrict-all-asm-resources" -DisplayName "Restrict All ASM Resources" -description "This policy enables you to restrict ALL Azure Service Manager (ASM, aka Classic) resources." -Policy '.\Restrict-ALL-ASM-Resources.json' -Mode All
$assignment = New-AzureRMPolicyAssignment -Name 'Restrict All ASM Resources' -PolicyDefinition $definition -Scope "/subscriptions/$subscriptionId"
i.e. when I tried to create a classic VNet, I could not pass the validation: