The S3 Block Public Access feature (free) allows you to block public access to all your Amazon S3 Buckets at the account level (in fact it is the default setting to avoid human errors).
Only by removing the lock is it possible to publicly share data from a bucket (for example to create a static website). This block even invalidates Bucket policies.
There is a visual marker with a yellow background and the word Public next to the bucket when you have a policy that allows public access.
You can read more about the subject in the AWS Blog