Hi Sara,
Unassigned is a valid and out-of-the-box option that is always available. It cannot be removed from the list.
This is one of these cases where it sounds like it could be addressed via user training especially considering that the worst case scenario is that the user will see a new error after the incorrect mapping and will have to map it correctly in order to successfully validate i.e.: no harm to the system/data.
From a technical point of view, you could, however, potentially track the save changes action and raise an error / exception using the WcfEventHandler preventing the unassigned option from being selected by, for example, a non-admin user.