Go to “Publish” and click on "Options”.
Click “open” and change the status of the survey. You'll be asked to confirm if you want to close the survey, click "Close this survey" then click "Save".
You can also close a survey from the My Surveys page by clicking on the small green circle to the right of the survey's title. The circle will turn red meaning that the survey is closed.
You also have the option to planning a validity period :
Go to the publishing module of your questionnaire and click on tab "options"
Under the Survey expiration date, check the box "validity period", choose your start and end date and save your options.