Go to the project manager and find the design you'd like to delete. Then click on the cog wheel icon and click on the "Delete" button.
Tip: If you're using the Desktop App you can simply select the project or the design and press Cmd + Backspace (Mac) / Delete (Windows)
If you accidentally delete a design, you can contact us and we'll renew it.
Avocode Web Manager works almost the same, but it is not possible to delete single Artboards from one Sketch page at the moment.