To take tickets off sale, go to the event management page and scroll down to the Ticket info section. Select the ticket(s) you want to take off sale and click the edit button next to it. Change the quantity available to 0, click Update and then click the Save button at the bottom of the page. Tickets will then be off sale - showing as Sold Out to the customer.
NOTE: Please note, changing the quantity to 0 will update the available tickets. If there are tickets in a customer's basket at the time of the update, these tickets will still be available for the customer to purchase. If the customer abandon the checkout, these tickets will go back on sale. If this happens you will need to update the quantity available again to remove these extra tickets.