|Perhaps you can clarify what you mean by "logged in." Do you mean users that haven't logged into your Domain in over two months? If so, why not just disable their accounts? No point in preventing access to a specific program if the computer isn't being used by those people in the first place. :)|
As for new users, I think the only thing you could do is set permissions on the Outlook.exe file, but you'd have to add all the users that are currently using it on each computer. I don't think it's possible to restrict by GPO...at least not based on your criteria.