Azure policies can be used to define requirements for resource properties during deployment and for already existing resources. Azure Policy controls properties such as the types or locations of resources.
Azure Policy is a service in Azure that you use to create, assign, and manage policies. These policies enforce different rules and effects over your resources, so those resources stay compliant with your corporate standards and service level agreements.
In this question, we would create a Azure policy assigned to the resource group that denies the creation of virtual machines in the resource group.
You could place a read-only lock on the resource group. However, that would prevent the creation of any resources in the resource group, not virtual machines only. Therefore, an Azure Policy is a better solution.
Reference:
https://docs.microsoft.com/en-us/azure/governance/policy/overview