I've seen a couple of posts, that had ads being served from elsewhere, and don't believe they have a case, if this is the case, it's just like linking to an image on another site, it would not constitute infringement...he's a snippet from another post..
Definition of "copy"
Several important rights exist under the United States copyright law only for “copies” of works — material objects in which the work is embodied.[13] Section 106(1) prohibits the reproduction only of copies of works, and section 106(3) prohibits the distribution only of copies of works.[14] Thus, as the Ninth Circuit held in Perfect 10, Inc. v. Amazon.com, Inc., a link (even a deep link or inline link) to an image does not involve reproduction of a copy of the image by the person on whose web page the link appears.[15] An instruction to a browser to jump to an URL is not a reproduction or distribution of a copy of what is at the referenced URL.[16]
http://en.wikipedia.org/wiki/Copyright_law_of_the_United_StatesI think that this means that actual copying and distribution of content are infringements. But, content that is only linked to on the Web is not considered an actual copy, nor is it a form of distribution.
I would point this out to them, and see where it goes.. Me being me and so fed up with this whole situation would probably threaten to counter-sue them if they don't give you something in writing dropping the whole matter, but thats just me.