This simple extension includes a block user function for comments and posts, as well as a downvote auto un-hider for subreddits that hide the downvote button with CSS.
To block someone, simply click the block button next to the post. To unblock, open the menu by clicking the Toolbox's extension icon, and select "unblock" next to their name.
(If Reddit isn't the current active tab, you may need to refresh the page after unblocking to see their posts again)
All feedback / suggestions are welcome.