If You're using the “junkmail@yourdomain” technique to teach your server junk, You can make the server auto-delete learned stuff like this:
$ sudo bash
$ export EDITOR=nano; crontab -e -u cyrusimap
30 13 * * * /usr/bin/cyrus/bin/ipurge -d 1 -f user/junkmail
and if “crontab -e” gives you grief (like hanging and not writing the file), then just edit /etc/crontab