Hi,
Each skin has a stylewebstore.css, though it doesn't really matter which files the css goes in, they are separate only for organization purposes but are all combined into 1 at runtime.
The main cart link in the store could be hidden I think with:
.cartlink { display: none; }
The add to cart links on the product list and product detalpage could be hidden with:
.offercontainer { display: none; }
Note that when you make a change to the css files, you need to clear the server cache by touching Web.config to recycle the app, and clear your browser cache, see more info here.
Hope it helps,
Joe