You are able to block off time for an employee if they will be out for a few hours during the day.

In order to get a prompt that the employee is not scheduled, go into File, System Options, Global tab and check the box for 'Use Employee schedule'.

Click on Schedule, Weekly Schedule.  Choose the employee name from the drop down and click the Padlock icon at the top.  Put in the dates/times for the block.  You can also add a reason if you'd like.

When you make an appointment for this employee at that time you will get a popup message.