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.

Code:
# 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] ...))
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 galsync@grstarbuc.com, but to no avail.

Finally I returned to the admin console and noticed that the galsync@grstarbuc.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.