... which I guess goes along with a way to not show a practitioner on the calendar before they start, but to show them thenceforth.... (until they too finish). If this is an "idea", as defined by this good forum, I'd "like" it.
There isn't currently a feature to show or hide columns only for select dates. Our recommendation for leaving practitioners is to use the Appointment Book - Rules to rule out their column and make it unavailable from the day they leave the practice. This will stop users from accidentally making a booking after this date, and once the practitioner has left you can then remove the columns entirely.