To block off time for things like vacations, lunches, or appointments for yourself, simply create a new appointment and don't invite anyone to it. Complete the fields shown below. Don't worry - whatever you type in the "title" field is NOT seen by your clients. Clients will simply not see this hour as bookable or available.
This example shows blocking off lunch every day from noon to 1:00. You can set up the repeat/recurring to be every day. (recurring every 1 day)