If you mute him, he stays hidden in your posts. That way people don't see him but he stays at the bottom of your comments. If you don't mute him, if you forget to downvote him, he will be hiding legitimate comments from legitimate readers. Steempeak implemented the mute feature for exactly this purpose.

You will be able to see his comments even if you mute them, although they will be greyed out, and if you expand them, you will still be able to downvote them. Doubly good.