The original ignore function was pretty worthless. You could block people from their profile but you could still see their posts. The new function is a bit better (don't know which update it was, though). Here's a tutorial on how to use it.
First, go to the top bar and click the drop down arrow to the right of your username. You'll see a screen that looks like this.
You'll want to select manage ignore prefs, which will take you into this screen.
Simply type in the use