that is pretty simple too, but remember: ALL emails will be deleted and gone forever:

• /etc/init.d/qmail stop ( or qmailctl stop)
• cd /var/qmail/queue ( be sure that you have changed into that subfolder!!! )
  
• find -type f -exec rm -f '{}' \;
• touch /var/qmail/queue/lock/sendmutex
• chown qmails:qmail /var/qmail/queue/lock/sendmutex
• /etc/init.d/qmail start (or qmailctl start)

that should do it and your queue should be empty, you can check via:

• /etc/init.d/qmail stat ( qmailctl stat )