Adding Criteria On The Field Being Filled.

I am working on a scheduling form that uses 3 combo boxes to select the people being scheduled. The form is rowsourced to a table of training sessions with a field for each of three crew positions being trained. One combo box is used for each of those fields. The row source for each combo box is a query that returns a list of people due for training, not on vacation that date, correct crew position, etc.

The problem is this: I want to eliminate the people already sceduled in prior sessions from the list. I've tried to put a "not" criteria on the rowsource query but that did not work.

I've been away from Access for a few years but I vaugely remember problems of putting criteria on the field you are filling. Anyone have any ideas?