1) Make everything in a bucket public (e.g. everyone has read access)
s3cmd setacl s3://myexamplebucket.calvium.com/ --acl-public --recursive
2) Make everything in a bucket private (e.g. accessible only to the creator)
s3cmd setacl s3://myexamplebucket.calvium.com/ --acl-private --recursive
The above commands also work on folders within buckets, or on individual files (the --recursive option is not required in this last case). As you can imagine there are many more options (type s3cmd --help to see more help).
3) Disable Directory Listing in a bucket
s3cmd setacl s3://myexamplebucket.calvium.com/ --acl-private
By default, new buckets will list all of the files within them. This isn't usually a good idea though, as it makes it very easy indeed for someone to scape all the content of your site. The command above just omits the --recursive option. For new buckets it's probably best to run command (1) before (3) so that your files are actually readable.