Someone is hacking my footer with a link to their site

Someone hacked my site and put a link to another site.

Please go to:
http://microbrewr.com

Scroll to the bottom and look at the footer section.
On the left column, I have my navigation menu.
Notice the tiny little square. It is this link:
http://www.sundayknight.com/

Which redirect here:
http://www.sundayknight.com/En/Index.Asp

I don’t recognize this. I don’t know how it got there. I can’t find it in my Widgets section.

It seems that someone hacked my site and put a link to their site to try and improve their search rankings?
WTF

How can we remove this?
How we prevent anyone from doing this in the future?
How can we reprimand the site that did this?

Thank you for your prompt attention to this matter.