Which combination of steps should be taken to proactively enforce the new process in the MOST secure way possible?

Explanations:

Ensuring all AWS accounts are part of an AWS Organizations structure operating in all features mode allows for centralized management and enforcement of policies, including Service Control Policies (SCPs). This structure is necessary to implement the governance required to enforce the new process.

Using AWS Config to report on IAM policy attachments does not proactively enforce the new process; it only provides reporting capabilities. While it’s useful for compliance monitoring, it won’t prevent users from purchasing or modifying Reserved Instances.

Creating an IAM policy with a DENY rule in each AWS account would not be a scalable solution. Each account would require manual updates to IAM policies, making it difficult to manage uniformly across hundreds of accounts. Furthermore, IAM policies are evaluated together, and if there are any Allow rules, they could override the DENY.

Creating a Service Control Policy (SCP) with a deny rule for the ec2and ec2actions is the most effective method to enforce the new process. By applying the SCP to each organizational unit (OU), the company can centrally manage permissions and ensure compliance across all accounts.

While operating in consolidated billing mode allows for cost management across multiple accounts, it does not inherently enforce compliance or governance related to the purchasing of Reserved Instances. It lacks the necessary controls to ensure that the new centralized process is followed.