The feature request sitting at the top of Sketchpad's UserVoice forum is for the ability to remove a sketch from your studio account to keep things tidy. I finally had a chance to build this in yesterday. From the list of your sketches, click through to view the sketch that you'd like to remove. If you were the original author (rather than just a subsequent editor), you'll be able to click a "Delete this sketch" link to remove it from your account. Un-clutter away!