When you block a slot in the calendar, you can choose what to display to customers on the booking form. Set these options by following the below steps.
Navigate to the settings option, and select the 'Booking Form' wizard.
Cycle onto the 3rd step and select the 'Slots' tab.
Scroll down to the option called 'When you want to block a slot the options you will see from the list'
Enter the option you would like to display e.g. Unavailable, Call us to book, Email to book.