Download the data from the existing S3 bucket to a new EC2 instance. Then delete the data from the S3 bucket. Re-encrypt the data with a client-based key. Upload the data to a new S3 bucket.

Block access to the public range of S3 endpoint IP addresses by using a host-based firewall. Ensure that internet-bound traffic from the affected EC2 instance is routed through the host-based firewall.

Revoke the IAM role's active session permissions. Update the S3 bucket policy to deny access to the IAM role. Remove the IAM role from the EC2 instance profile.

Disable the current key. Create a new KMS key that the IAM role does not have access to, and re-encrypt all the data with the new key. Schedule the compromised key for deletion.