First log in to your .tk account.
Once logged in, you need to choose from the links menu on the left, "Dot Tk Linker Promo". After clicking this, it will take you to a page oin which you select your domain (im not sure if this page is included if you only have one registerd domain). After selecting the domain you wish to remove the banner, click the CONTINUE button
On this page that you are on now there will be 3 radio buttons titled "No Banner", "PopUp Banner" and "Frame Banner" - make sure "No Banner" is selected and then scroll to the bottom of the page and click CONTINUE
After clicking Continue the page will seem as if it has refreshed and a green highlighted box will appear at the top saying it has been disabled! - now give it a few minutes to update on your site and then it will be gone!