On the Page & Survey tab, head to the Survey Questions and Dynamic Content section.
Go to the question for which you wish to set a close date and click the Edit gear.
In the next dialog, click Options.
In the popup, select Close Poll.
When you return to the question, it will be marked as Closed in red at the bottom right of the question box.
Not what you were looking for? Check out our article on How to set a close date for receiving input.