[Dovecot] prohibit deletion of Junk folder

Hi,

I have an issue with a sieve_before rule (the script move incoming email to Junk folder when DMARC=fail and policy is quarantine) because one of my user delete his Junk folder :

dovecot: lda(myuser@domain.com): Error: sieve: execution of script /etc/dovecot/sieve_before/01-dmarc-fail.sieve failed, but implicit keep was successful: 1 Time(s) dovecot: lda(myuser@domain.com): Error: sieve: msgid=20131124212217.5F346580D49@anotherdomain.com: failed to store into mailbox 'Junk': Mailbox doesn't exist: Junk: 1 Time(s)

How can I do to prohibit the deletion of Junk folder by end users ? Tell me if you need 'dovecot -n' and thanks for your advices.

Alexandre.