In the Admin Console, click on 'Manage Orders' and then 'Current Open Orders'.
You'll see two dropdown fields at the top of the screen. Select the day and the time you want to block and click on 'Reserve Timeslot'. This will reserve one timeslot for that time.
If you only accept one customer per timeslot, then this timeslot is now blocked to all customers ordering online.
if, however, you accept 2 customers per timeslot, then it will reserve one of those two positions. Another slot at the same time will still be available for an online customer.