The receptionists are asking for a way to force appointments into a full time slot - apparently it's something they frequently do.
I suggest we do this by adding a checkbox to the create appointment form called "show full time slots." If checked, full slots should appear in the table in red. If the user selects this slot and tries to save an appointment into it he will be given a dialog "are you sure you want to make an appointment into this full time slot? There are already XX minutes of appointments in a XX minute time slot and this appointment will take an another XX minutes." with OK and CANCEL buttons. We'll have to work out the exact phrase. If OK, save the appointment. If CANCEL, return to create appointment form.
I think this ticket will also satisfy