How can I remove Google's cache of all of my threads in my forum?
 

I'm not wanting to remove Google's indexing of the threads in my forum - just their cache.

I have a lot of images in my threads that I want people to have to come to my forum in order to see them.

I know how to remove the cache of individual threads using Google's webmaster tools. But what folder can I tell Google to delete the cache from?

And what is the proper meta tag to use for all robots to index the threads but not cache them?

Thanks