From Test-Scratch-Wiki

To delete a project from the website, go to your My Stuff page (make sure that you're logged in). Scroll down until you see the projects that you want to delete. Click "unshare" next to the project, then click "delete" To delete projects from your hard drive, delete the .sb or .sb2 file like any other. To permanently delete it, go to your trash and click the Empty Trash button, then confirm your password. And remember, what is emptied is now permanently gone!

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