It's likely that the ads are coded tight into the theme itself, you'll need to edit them either with the WordPress Admin editor or offline with a proper text editor. Chances are the ads are in the sidebar.php or page.php files, but without having a copy of the theme it's hard to say for sure.

In any case I would make sure to keep a copy of the unaltered theme handy in case you mess anything up.

Bill

Contact me at matt@snidge.com and I'll help you out. If you would like to attempt this yourself, you need to login to your FTP program and access the .php files. Make sure to back things up. As mywebwork pointed out, it's probably coded right into the theme.

If it was a free theme that came with a Terms of Use, check to make sure you aren't required to show someone else's ads. It's a dumb requirement, but some theme authors peddle their freeware that way.

Check the Plugins tab, in the admin panel, and the Appearance->Widgets tab to see if a plugin or widget was installed along with the theme. If the theme is designed to allow you to place your own ads, there might be an included tool for adding them.

Finally, still under the Appearance link in the admin panel, click Editor and review the list of files that appear on the right-hand column. See if one of them had the word 'ad' or something similar.

Otherwise, the ad code is in the template as the other posters mentioned. If you have experience editing, you can find the right file and comment that code out.

You can do that directly from the WP theme editor, but make sure you have a backup of that theme directory first in case you break the code.