A+ time slot configuration

Hi, I need some help please.

I have a 90min service. A time slot is available for example at 1.00pm but I have already an appointment booked at 2.00pm.
Time slot at 1.00 is not available for a 90min service as only 60 min is available on my schedule due to my next appointment.. I can see that it's impossible to book that service once you try to book it. Perfect.
But how to make disappear the 1.00pm time slot or make it busy on calendar to prevent guests trying to book that appointment instead of letting believe that this slot is available?
Don't know if I am being clear..
Thanks