Closing registration before an event starts

Select the Events application from the Apps icon in the top left of the window.
Click the Organization Events tab.
To Edit an Event: Click on the Event Name for the event you wish to edit and then click the Edit button.
Set the Registration Deadline Days setting:

Registration Deadline Days Before Event: Sets the number of days before an event on which registration should end. Registration will close at 11:59 pm.
If the Registration Deadline Date is 0, registration is allowed until the end date/time of the event.
If the Registration Deadline Date is greater than 0, this allows registration until the number of days prior to the start of the event.