If you are, go into your Account Manager and you should see it. I have a better suggestion though...just don't open their posts!

Can't tell by your post. Euther way it's simple.
If you want to ignore someone, then just click "ignore user" at the bottom if any of their posts. Although I never understood the reasoning with this. If you don't want to read someone's post, don't read it! I fail to see the need for a button to do that.
And by the way you can't ignore aliases.

Now if you are trying to unignore someone you inadvertantly ignored (it happens. I've done it when I am on my smart phone, easy to hit the button by mistake)
3 sime steps:
Click on "mail"
Click on "ignore list"
Click on "remove"