To prevent appointments being requested during lunch hours in OpenDental: 

Since CareCru retrieves the hours from your OpenDental system, all you need to do is to schedule your lunch hours in OpenDental, and Donna will know not to offer appointments during that time.

Here is a quick excerpt from the OpenDental guide below:

On the appointment schedule, right-click on a time block in an operatory, then click Edit Blockout or Add Blockout. Edit Blockout is only enabled when you right-click on an existing blockout.

Start/Stop Time: Both times default to the time clicked on the schedule. Manually change times or click the drop-down to select an option. You are not required to use 30 minute increments. If manually entering, use the format 00:00 AM.

Note: Optional text that shows under the blockout label. This text box has right-click options (Right Click Text Box).

Blockout Type: Determines the blockout text, scheduling and editing preferences, and color. Customize options in Definitions, Blockout Types.

Operatories: The operatories the blockout will apply to. To select multiple Operatories, press Ctrl while clicking. If using Clinics, only operatories assigned to the clinic selected in the main menu are options.

When blockouts with the same type are added to multiple operatories at once, they become connected. Deleting or replacing a blockout in one operatory will also delete or replace any blockouts that are connected.

Full Guide at : https://www.opendental.com/manual/blockouts.html