A simple solution is to mark yourself as busy then IMs dont get through. As for making it so "friends" cant see you if they are in a group you belong to they can see. Good friends respect your privacy if they do it once just say hi sorry i am busy at the moment ill catch you later, they get the point
The IM's from individuals at least get through to me. I think on busy you don't see some group IM's and offered inventory may go to trash, but I sure saw the IM's from my nosy former friend when I set busy.