General :: Saving Records And Empty Comboboxes

I have a database that I've created for a number of clerks under me to use to track their daily tasks.

When clerks are entering their tasks, they do so through a number of combo boxes on a form (Assigned by:, Division of Law:, Subcategory:, Casename etc). When all of the combo boxes have been filled, the employee can click a button in order to save the record to the back end of the database and open a new blank record.

Currently, if an employee tries to enter the wrong data in a combo box and save the record, they will be prompted to enter data and not allowed to save the record; the problem is that if they leave all fields blank they can still save the record - thus creating useless data for reporting purposes. For example, an employee could enter their name and the date of record but leave every other combo box blank and still save the record. I want to lock the form such that they cannot save the record unless all of the combo boxes on the form are populated to reduce user error.

How can I do this simply?