Immediately revoke all access to the S3 bucket and re-evaluate the IAM policies for the entire AWS environment.

Notify the affected users and instruct them to cease any activities related to the S3 bucket.

Conduct a thorough investigation into the cause of the unintended permissions and update the IAM policies accordingly, while also implementing additional controls such as bucket policies and access control lists (ACLs).

Increase monitoring of the S3 bucket for any suspicious activities and prepare a report for the security team.