Actually, you can -- I found this tip in the Canvas Guides (don't ever forget what a great resource they are!!):
To create single spacing in the Rich Content Editor, press the shift+enter (or, if you are on a Mac, hit shift+return). This will create a line break instead of a paragraph break.
To create single spacing in the Rich Content Editor, press the shift+enter (or, if you are on a Mac, hit shift+return). This will create a line break instead of a paragraph break.