To archive a form, you look for the correct form and click on the pull down menu on the right of this form. After that, you click on Archive in the pull down menu. A screen appears asking you if you are sure that you want to archive this form. Here you can cancel or archive. If chose to archive the form, it will be placed in your archive!
This archive can be found by clicking on Forms. If you look beneath My feedback form you will see Archive as well. By clicking on this you will see the archive overview.
To delete a certain form permanently, you have to follow the first steps explained for archiving. First you click on the pull down menu behind the form and then click on Archive. To permanently delete the form, you have to go to your Archive. After that you'll have to look for the right form and choose wether you want restore this or delete is permanently.
Want to keep the form? Just click on Restore behind the form.