The easiest way to follow or unfollow someone is via the four icons that appear when you move your mouse over someone’s photo. (Check out this post if you’re not sure what I mean.)
Click once on the the bottom right icon, then once on the User menu, and you’ll see the following submenu appear:
From here you can follow, unfollow, block, block & report spam.
Follow means you’ll be receiving all their tweets; unfollow means you won’t.
(Blocking is for those people you really don’t want to have anything to do with, such as spammers.)
[...] Tweetdeck: Following, Unfollowing [...]