BoldGrid does not have a dedicated button to turn this link off. This is to make sure that an effort is made to provide attribution for content used freely from third parties, including code and images. Imagine you were a photographer or coder and allowed your work to be used freely and only asked for credit. That’s why the attribution page and Special Thanks link is included.
However, since BoldGrid is open-source, you can modify the code to hide those links should you desire to remove all attribution, but there is not an on/off button in the menu itself.