I suppose you could "wrap" the form field in an if statement that evaluated the participant field to see if anyone has taken the time slot, then if someone has, it becomes read only. However, this would not allow the user who signed-up for the time slot to remove themselves (admin only).
Also it does not prevent someone from making changes to a participant field if they have a good understanding of html forms and can submit a modify_record string manually.
Also it does not prevent someone from making changes to a participant field if they have a good understanding of html forms and can submit a modify_record string manually.