So you want to schedule lunch and ensure that you won't have any conflicts? Or you know you're going on a grand vacation and won't be available? No problem!
To schedule a break, simply add an event to your calendar (recurring if desired) that spans the time you would like to block out. This will prevent your clients from scheduling events at that time.
For a vacation you can do the same by ensuring the event is busy and spans the full length of your vacation.