This is just an FYI since I didn't find an answer to this problem when I originally searched the forums.
This morning I made a typo when adding a domain via the admin interface. Yeah, I know. Anyhow, I first tried to delete the domain from the admin panel, and then using zmprov. They both failed, apparently because there are still resources attached to the domain.
Reading the zmprov message a little closer revealed that there was an "account" galsync@s=grstarbuc.com still in the LDAP database. I tried to delete the account using zmprov da firstname.lastname@example.org, but to no avail.
# zmprov dd grstarbuc.com
ERROR: account.DOMAIN_NOT_EMPTY (domain not empty: grstarbuc.com (remaining entries: [uid=galsync,ou=people,dc=grstarbuc,dc=com] [zimbraDataSourceName=zimbra,uid=galsync,ou=people,dc=grstarbuc,dc=com] ...))
Finally I returned to the admin console and noticed that the email@example.com was a special account, however I was able to delete it like a normal account.
This fixed my problem and I was able to delete the domain.