How to REMOVE the BIG SIGN-UP HEADER for not logged in users on EVERY page

Buddypress - social theme. ..

How do I remove the big sign-on box/header on all pages for logout visitors?

I know the question have been answered before by Mason:

I need a more specific answer. I understand that you get the header.php from social theme and put it in the childfolder, but exactly where and exactly which code do you remove or edit?

I could not find a topheader.php in the social theme. Does it exist or is it in another folder? If it is needed, what do I do with that file?