The error means that an object (website, user, DNS-Record, etc.) is assigned to this reseller and it cant be deleted before this object is deleted.
1) Have you deleted all sites that belongs to this reseller?
2) Every reseller has its own trash folder, check the trash of this reseller if it is empty, i not empty it.
As domino described it may take some time when you delete an object until you can remove it from the trash, because ISPConfig has first to write the changes to the config files before it can be removed from the database.