I installed the dovecot-imapd package in Ubuntu, and my setup is as follows:

$ sudo dovecot -n
# 1.2.9: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-27-generic-pae i686 Ubuntu 10.04.1 LTS 
log_timestamp: %Y-%m-%d %H:%M:%S 
protocols: imaps
login_dir: /var/run/dovecot/login
login_executable: /usr/lib/dovecot/imap-login
mail_privileged_group: mail
mail_location: maildir:~/Maildir
mbox_write_locks: fcntl dotlock
auth default:
  passdb:
    driver: pam
  userdb:
    driver: passwd

For security reasons I would like to deny all users that are in the admin group ability to do imap login via dovecot. This is done in order to prevent a brute force attacker from discovering the admin passwords, and obtaining administrator privileges on the system.

How can this be achieved? Presumably, I will have to modify some settings in /etc/dovecot/dovecot.conf, but I am hesitant to change the default settings lest I create other security vulnerabilities.