You should consider applying the mfcheck patch to your qmail source tree and run make setup check after the patch is applied. This patch will cause qmail to do a reverse DNS check on the sending mail server to be sure it's got reverse DNS properly setup. if the mail server sending the messages are run by spammers, usually they will not have reverse DNS setup properly, whereas "real" mail servers will have it setup correctly.
The reason for all of this is so that if you DO receive spam and it's addressed to a bogus account on your system, you will be able to bounce the message back to the proper machine. This will reduce the number of bounce messages your system is unsuccessfully trying to deliver.
Also be sure to run...
echo 1 > /var/qmail/control/mfcheck
to enable the reverse lookups.
If you're getting triple bounce messages, this means something is messed up on your system. qmail normally disacards triple bounce messages automatically, but if you're seeing this message somewhere in your logs, it means there is not an adequate account for receiving the triple bounce messages.
If you'd like to simply discard the double bounce messages completely, you can run these commands...
echo 'doublebounce' > /var/qmail/control/doublebounceto
echo 'hostname.yourdomain.com' > /var/qmail/control/doublebouncehost
echo '#' > ~alias/.qmail-doublebounce
chmod 644 ~alias/.qmail-doublebounce
Be sure to change the host to your domain name.