Is it one page or can you have more (ie home page, contact, about me, etc) If you can hav more than one, just use
HTML Code:
<a name="start">
at the place you want to be the top of your page. Then all your internal links should be something like
HTML Code:
<a href="contact.html#start">
The page will then jump to the part that you want to be the top of the page, thereby cutting off the top ads section. You can't really lock the scrollbar into one place, at least not that I know of. So if a person chooses they can scroll back up and see the ads once the page has loaded