@Raymond8If you're viewing the calendar from a PC web browser, click on the first day and then click and drag the handle bar (a circle with 3 verticle lines) on the right down to the bottom of the month. Then simply click blocked.
On your phone (I can only speak for iOS, no idea bout Android) you have to tap tap tap each day in the month and tap blocked.
You can also limit how many month in advance guests can book. I keep mine to 3 months because I travel often and can't take plans too far in advance.