If you have one or more addresses that do not exist but are constantly getting email there are a couple ways to handle that situation.
What are you using to manage the domains and accounts on the qmail server?
The best option is to enable valid recipient checking so that email to non-existent addresses is rejected by qmail-smtpd before it's accepted into your server. Some of the implementations are mentioned here.
If you have a simple qmail install with no virtual domain management software, and it's always the same addresses getting hit with email, you can have qmail quietly, and more efficiently than the default setup, discard the email by creating .qmail files for those usernames with the only contents being a #
> cat .qmail-username
A .qmail file with a # in it means silently delete all email to that address.
If you want to prevent bounces back to certain recipients, depending on what domain name(s) they are in, you could configure your qmail to treat those domains local, or the better solution might be to blacklist email To: those addresses in your outbound spam filtering. Do you have the ability to blacklist and trap/delete on email leaving your qmail server?