Force a user to use POP3

[John Doe Vecot]

[dovecot-2.3.2.1]

I have dovecotrunning icw postfixandhave IMAP and POP3 available to 
retrieve mail.
Can you tell me how I can configure Dovecot in order to force certain 
users to use POP3 (download messages) only?

Thanks,
Jos