should be simple enough, but not enough info provided by you to be specific. So generally, you'd create a sql statement or query that filters out the ones that haven't been scheduled. If that's a Yes/No type of field, then you return those who are "no". If it's a date field, you return those who have no schedule dates set. Then set the row source of the combo to be that query, or copy the sql statement into the row source field on the property sheet.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.