You cannot define a deadline for registration for a special event.
However, you can set the deadline for yourself and deactivate registration for the event manually once your deadline has expired.
Go to: Overview => Settings => Manage Events and Items
Choose the relevant event from the list and click “Edit Event/Item”. At the bottom of the page you will find the option “Deactivate”. Check the box “The event shall no longer be selectable”, and save your new settings. Participants will now no longer be able to register for this particular event.