Blocking a time slot

From the daily view of the back-end calendar click over a row corresponding to the desired hour you want to block, click on "Block" button then select another row corresponding to the ending hour of the slot you want to block then click on "Block these rows".

You'll see these rows became grey as they are not more available for new reservations on front-end booking form.

To "unlock" and make them available again just click on "Unlock this row" button.