Does the mark appear when you print your document or is only on the screen?

You say you've tried to fix this but don't tell us how you've tried. Have you tried editing your headers to remove the mark? If so, have you tried doing it with field codes revealed (Alt-F9)?

Here is much of what I know about this:
Sections / Headers and Footers 2010

Why so many sections? If you are adding sections to change your header/footer text, consider using the StyleRef field instead.