For preventing appointments, you have a few options.
If it's just a day or part of a day, you can use a calendar block. Go to your calendar > tap + > tap block. You can use calendar block when you have your appointment at the dentist office so you can't take appointments from clients. If you're unavailable maybe every other Monday or something more consistent, you can use recurring blocks and delete the days you can still take appointments.
In the case your business is always closed on Sunday, you can be sure to have Sunday off on Settings > Scheduling
If a service is only offered during specific hours, you can specify the hours under the Settings > Services > Service Name > Schedule > Availability.
If you need to block specific time slots within your business hours, use the Block out time function by tapping ‘+’ from the calendar page