Their profile will become private so that you can not read their information or send them message. And whenever you try to post opinion on their questions or myTakes or try to comment on their opinions or vote them, you'll get this notification. s28.postimg.org/kl4dcpal9/2015_06_02_15_54_57.png
If you try to send them a message or comment on one of their Takes/questions you'll get a message saying "Not allowed to post! You blocked the user or are blocked by the user!" Across the top of your screen in red.