How to Allow for Single-Spacing in Text Editor?

Hello,
The content editors on my site need a way to show an unordered list (or paragraph with breaks) as single spaced. The default style on the site is 1.5 em. I know how to easily do this editing the HTML of the text, but am wondering if there is an easy way to allow them to make a block of items single-spaced using the WYSIWYG editor in wordpress. Thank yoU!!