main.inc.php
// store spam messages in this mailbox
$rcmail_config['junk_mbox'] = 'Junk';
// display these folders separately in the mailbox list.
// these folders will also be displayed with localized names
$rcmail_config['default_imap_folders'] = array('INBOX', 'Drafts', 'Sent', 'Junk', 'Trash');
Replace 'Junk' by 'spam'.