How to stop cache on specific folder?

That can be specified in the ‘Advanced’ tab, if I’m not mistaken. Look for the following text on that page:

Add here strings (not a filename) that forces a page not to be cached. For example, if your URLs include year and you dont want to cache last year posts, it’s enough to specify the year, i.e. ’/2004/’. WP-Cache will search if that string is part of the URI and if so, it will not cache that page.