Resources > All Venues - Select venue you wish to make private. Click to open the venue.
Click "Edit" button.
Locate "Venue Details" section. Locate "Client Selection" field. Select "Hide From Client" from drop-down box. Click "Save" button.
Suggestion: Set an automated procedure for newly created venues. See: Setup > Gigbuilder System > Event Defaults: Event Defaults section: Venue Visibility field.