You can close ticket sales in the Promotion Centre. Select the event you would like to close ticket sales for from the dashboard. Click the Manage Tickets tab on the left and View Tickets.

Locate the ticket you would like to close tickets for a click the Cog icon. Press Close.

For events with multiple tickets on sale, using the checkbox you can select individually / all tickets, press the bulk edit button.

Select Close tickets and Update Tickets.

To reopen a ticket follow the same steps but press reopen.