[markbolyard]

In your templates, you will see a section of templates called Ad Location Templates.

Double click that to expand the template section. That ad spot is called ad_header_logo

Revert that template, or simply delete it's content, or replace with your 468x60 banner, and save.

Note: All that ads are located in these templates:

ad_header_end
ad_header_logo
ad_navbar_below
ad_showthread_firstpost
ad_showthread_firstpost_start

I suggest you simply delete the adsense portion only, that way if you ever run ads, you will still have the style part of the template so it displays best.