To block someone, click on their name to pull up their Community Profile, click the little down arrow next to the "Follow" button and then select "Mute." This will hide their comments and posts, and prevent them from seeing any of your posts or comments.
To unblock someone, open the Community tab, click the three horizontal lines in the upper righthand corner, and select "Muted users." From there you will be able to unblock anyone you previously blocked.