You may have a couple serviceless MBRs that are still in this group.
You can't remove a billing group unless there are no MBRs with that group
Run the following script (changing "other" to whatever the groupname is that
you're having a problem with):
from masteraccounts ma, groups g
where ma.groupid = g.groupid
and g.groupname = 'other'
Then go into the accounts and change the billing group to some other group.
Once this is done, you should be able to delete the billing group.
For more information about this list (including removal) go to: