Flushing the mail queue with postfix is straightforward enough:

# postfix -f

But qmail, as usual, is a little more obtuse:

# ps aux | grep qmail-send
root     18971  0.0  0.0  1328  260 pts/1    S    18:18   0:00 supervise qmail-send
qmails   18978  0.0  0.0  1416  384 pts/1    S    18:18   0:00 [qmail-send]
root     19469  0.0  0.0  1724  604 pts/1    R    18:20   0:00 grep qmail-send
# kill -s ALRM 18978