Go to Event Tickets > Modify Tickets > Click On Ticket Name > Mark as sold out > Yes. You can also add a sold-out message that will appear on the ticket page. 

To view what it looks like go to Event Tickets > Launch Ticket Page Or view the screenshot below. 

* Once you create a Ticket/Sponsorship/Underwriting and a purchase or sale has been made, do not change the name of the ticket. By changing the name of the Ticket/Sponsorship/Underwriting after a purchase has been made your reports will no longer be correct and the resent guest link will not show for Tickets/Sponsorships/Underwriting.