You could try keeping commented as first line in .qmail-default the same line usually written by qmailadmin inside .qmail-default when bouncing is enabled.
Or you could use chkuser patch, enabling it always or adding a commented line, containing "bounce-no-mailbox" to .qmail-default (see www.interazioni.it/opensource <http://www.interazioni.it/opensource>).
Thanks! chkuser solved the need to reject non-existing users...
Best regards, Karsten