You can delete it from its page, with the Delete this form button. Can Do will ask you for confirmation: "Delete this form? This will also delete all submissions."
This cannot be undone. The form and all its response history saved in Can Do are deleted, and the public link stops working immediately.
What was already created in Basecamp from previous responses (to-dos, Card Tables, Messages or events) is not deleted; deleting the form in Can Do does not undo what is already in Basecamp.