It is possible to remove the "Buy this book" link from the Google’s Book Search feature.  You must contact Google support and ask them to remove "third party Buy links" for your URL.  You also need to add this to your Book Search code:

<script>GBS_setViewerOptions({showLinkChrome: false});</script>

directly following:

<script src="http://books.google.com/books/previewlib.js"></script>