Well i dont think they will ban you for that coz first of all they have put that seprately in the code if they wanted it to be compulsary they would have kept it in the main code
From the code, it is possible to remove it... But if allowed, you need to read the TOS or contact adbrite... But why do? Some user might wanna advertise on your site, so that small link may earn you extra
You can...Remove this codes, located the bottom part of the code. <a target="_top" href="http://www.adbrite.com/mb/commerce/purchase_form.php?opid=xxxxxxx&afsid=1"><img src="http://files.adbrite.com/mb/images/adbrite-your-ad-here-leaderboard.gif" style="background-color:#FFFFFF;border:none;padding:0;margin:0;" alt="Your Ad Here" width="14" height="90" border="0" /></a></span>