Select the Smoovie, scene, frame or effect which you want to delete and then either select Delete from the Edit menu or press Cmd+Backspace (⌘⌫) on your keyboard as you would to delete a file in Finder.
We've been asked why we haven't made this feature more prominent, for example by adding a delete button beside the add button. The answer is that we didn't want to make it easy to delete content by mistake.
An alternative was to have a 'Are you sure you want to do this' message, but, really, do you want us to pester you with these messages all the time?