If you have the need to forcefully delete everything in the qmail queue, simply run this shell script (thanks to Florian on this one):
#!/bin/sh
# remove everything - STOP QMAIL FIRST!
/sbin/service qmail stop
for i in bounce info intd local mess remote todo; do
find /var/qmail/queue/$i -type f -exec rm {} \;
done
/sbin/service qmail start

Thanks, that was handy. Cheers!
I needed this badly. Thank you.
My only suggestion would be to modify the script so that the file names are printed as they are deleted. It would help to have a clear indicator the script is working.