How can I reject sync changes for my OS X Contacts?

How can I reject sync changes for my OS X Contacts?

I have my OS X Mountain Lion contacts hooked up to synchronize with Google
Contacts. Every once in a while, I receive a notice that "Syncing with
Google Contacts will change more than 25% of your contacts or groups on
this computer."
Almost always, this just means that the images are mysteriously being
removed from my contacts. My only two options are "Sync Later" or "Sync
Contacts". I would prefer to mark my local contacts as master contacts, or
to be able to reject the proposed changes. How can I do this? (Short of
manually and laboriously exporting my local contacts and uploading them to
Google Contacts every time it has a few changes I don't like).