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 achieved, you just need to click the 'Archived Questionnaires' below your active questionnaire’s grid on the Customization section, and choose 'set active'.