This applies to Zimbra 8.8.8+. To delete all outgoing queue for a user, execute the following command with root privilege:
/opt/zimbra/common/sbin/postqueue -p | awk 'BEGIN { RS = "" } { if ($7 == "user@example-domain.ma") print $1 }' | tr -d '!*' | /opt/zimbra/common/sbin/postsuper -d -