The solution requires two actions: immediate remediation and long-term prevention. First, enabling Amazon S3 Block Public Access on the bucket or account level immediately removes all forms of public access. This feature overrides any conflicting bucket policies or Access Control Lists (ACLs). Second, to ensure this setting cannot be reversed, a Service Control Policy (SCP) must be applied through AWS Organizations. By creating an SCP with a Deny statement for the s3:PutPublicAccessBlock action and attaching it to the relevant account, you prevent any IAM principal (including the root user) within that account from modifying the Block Public Access configuration. This combination effectively meets both requirements.

A. KMS encryption protects data at rest but does not control network access. Denying s3:GetObject via SCP would block all read access, not just public access.
B. While enabling Block Public Access is correct, denying s3:GetObject via SCP is an incorrect preventive measure as it blocks all object retrieval, which is overly restrictive.
D. Object Lock is a WORM (Write-Once, Read-Many) feature for data immutability and retention; it is not related to managing public access permissions.

1. AWS S3 Developer Guide, "Blocking public access to your Amazon S3 storage": This document explains that S3 Block Public Access provides settings to manage public access to S3 resources. It states, "Amazon S3 Block Public Access can override ACLs and bucket policies so that you can enforce uniform limits on public access to these resources." (See section: How Amazon S3 Block Public Access works).

2. AWS Organizations User Guide, "Service control policies (SCPs)": This guide details how SCPs act as a guardrail to control permissions in an organization. It clarifies, "SCPs are JSON policies that specify the maximum permissions for an organization or organizational unit (OU)... SCPs limit permissions for IAM users and roles in member accounts, including the member account's root user." (See section: Introduction to SCPs).

3. AWS IAM User Guide, "Actions, resources, and condition keys for Amazon S3": This documentation lists all IAM actions for S3. The s3:PutPublicAccessBlock action is defined as granting permission to create or modify the PublicAccessBlock configuration for an account or a bucket, confirming it is the correct action to deny for prevention. (See the Action summary table for Amazon S3).

4. AWS S3 Developer Guide, "Using S3 Object Lock": This source describes Object Lock as a feature to "store objects using a write-once-read-many (WORM) model," which is unrelated to controlling public access. (See section: How S3 Object Lock works).