We provide you with unlimited storage in your archive so we don’t recommend deleting anything, but sometimes you just have to get rid of stuff.
If you really want to permanently delete a video, look to the left of the projects screen, where you’ll see your name. To the right of that is the folder name, and to the right of that are three icons. The third one (with the trash can) is your ‘delete’ button. Click this and the folder will disappear, leaving no trace of its existence.