If Employee Already Selected Do Not Allow To Be Shown In Drop Down Again

I am trying to create an available list of employees to assign to an event. Basically, when I assign someone to the event and need to assign additional people, I don't want the ones who have already been selected to show up in the drop down.

Sheet1 is the linked Excel sheet where all of the employees are contained.
Assigned officers is the table where those selected to work are stored.

Using the following code, I get a prompt for sheet1.[employee id]

Code:
SELECT Sheet1.[Officer Name], Sheet1.[Employee Number], Sheet1.[Employee Number]
FROM Sheet1
WHERE (((Sheet1.[Employee Number]) Not In ([Assigned Officer].[Employee ID])));
;

I don't think the "not in" is correct.