There isn't a direct option to do this, but I can recommend a workaround.
1. Go to your Bookings module in Office 365
2. Click on "Calendar" on the left column
3. Select "Time off" on the top ribbon
4. Select all the staff members (make sure they have a "check")
5. Select the Start/End dates for the time-off - I would recommend that you block the time after your last day of availability until either i) you have the service available again or ii) set a date far enough in the future that no one would look that far away
7. Go to "Booking page"
8. Click on "Save and publish"
9. Click on "Open published page"
After those steps, when you go to your Bookings published page, you shouldn't be able to book any appointment during the "Time off" dates you specified.