The Bugzilla schema has had a field named "disableduser" in the profiles table since at least v2.10, i.e. almost every instance in the field has this field. When a user is marked as disabled in Bugzilla, some text is entered into this field. This information could be used to create the user differently when importing data from Bugzilla into JIRA. For instance, the email address could be made invalid in an RFC-approved manner, the user could be created but not added to jira-users.

The business case for this is that it's a right pain to have to deactivate hundreds of users by hand after the import!