You should go to the homepage of your dashboard and pick the distribution button of the questionnaire you want to deactivate.
Then, you will be redirected to the distribution page.
Go to Web Integration, and turn off the Active Button!
By doing so, you will not collect responses for this questionnaire. To activate it again, follow the same steps.
Important Note: Don't delete a questionnaire because all the responses will be lost. If you want to delete a questionnaire,first download all the data related to this questionnaire and then you can delete it.