Remove selections from a cross relationship drop down field

I am building a scheduling app and im having difficulty finding a way to limit the # of people who register per time slot. My table structure is:

    [Days] > [Time Slots]
          v   v
   [Scheduled Appointments]

I have a mechanism to stop users from saving with an oversubscribed timeslot but id like to remove the time slot from the drop down when it too many people have signed up


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GMacri
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