Once accepted as an expert, you will find additional menu items available to you.
Under Profile you will see 2 tabs called Recurring Availability and Availability Calendar. Your Recurring Availability is how you can set your availability that will repeat every week going forwards. The Availability Calendar is the way that you can set specific time slots on any specific date. You will notice that adding some Recurring Availability will reflect immediately on the Availability Calendar.
To cancel your availability on any given day or time slot, you need to enter the Availability Calendar and click on the X at the top of that time slot. It will ask you for confirmation that you wish to cancel this availability.