By default, lunches are marked as locked. However, should you wish this to be just a 'Reserved' appointment, you can do so through the Users tab in Maintenance.

Click the 'Edit' button next to the user you wish to change their lunch set up.

Then in the section that displays Start/Finish and Lunch Timings, tick or untick 'Lock Appointment Lunches' as preferred.

Now when adding a clinic for this user in the diary, their lunches will either be Locked or Unlocked as you have selected.