When calculating the quota of a maildir, qmail-ldap ignores any mail that has been trashed. While this meets the design of maildir++, it is, in my opinion, a bug in the specification, as users can keep more mail than their quota allows by trashing it without expunging.
The following patch disables this behaviour.
The patch: is here Current version is qmail-trashquota-1.03-20020901-20020909. It is to be patched against qmail 1.03 and qmail-ldap 20020901