Hover your mouse on the right side next to a post and a little blurb will appear saying "Hide person or Hide *insert particular app or game*". Click on it and you can hide it and also unhide it whenever you want.

"Hover over the application story in your News Feed and click the "Hide" button. Selecting the name of the application from the drop-down menu will prevent all stories from that particular application from appearing on your home page. Remember that selecting the name of a person from this menu will hide all stories from that person."