How can I delete a questionnaire?
It's not possible to delete a questionnaire, because then all the matters attached to it would become inaccessible.
But you have the option to archive a questionnaire. To archive a questionnaire - choose 'Archive Questionnaire' on the questionnaire you would like to hide on the Customization section:

If you need to unhide a questionnaire you archived, just click 'Archived Questionnaires' below your active questionnaires grid on the Customization section, and choose 'set active'.

Frequently Asked Questions
Why can't I delete a questionnaire?
Deleting a questionnaire would make all the matters attached to it inaccessible. Archiving hides it instead, while keeping those matters intact.
Can I bring an archived questionnaire back?
Yes. Click Archived Questionnaires below the active questionnaires grid on the Customization section and choose Set Active.