Hi @Skallio9

While there's not a setting that you can enable to do this easily, there are two ways that you can accomplish this.

You can use the "Disable Online Booking Site" option in the Visibility & Display tab of your Appointments Dashboard. This will prevent your customers from taking any actions from the online web page. 

Another workaround that could help would be to create a recurring personal event with the "All Day" and "Block" boxes checked. You'll set the start and end date for the first and last day of the month that you don't want to allow bookings.