I would start out with the "what do you need?/can I help you?" routine. And when she says "nothing", stare at her till she turns around. And do it every single time.
If a couple of days of doing that doesn't get her to quit doing this, at least to you, then I would talk to her along the lines of what Kaypeep suggested.
And if that still doesn't work, well, I'd be blunt and just say, "This conversation doesn't concern you so please stop listening in and go back to your own work." Again, every time, and stop your own conversation until she complies.
As far as looking at shirtless guys, I'd definitely report that to my supervisor and hers. It's completely inappropriate office behavior and can get her and the company in trouble if she does this with the wrong person. If the supervisors ignore it, run it by HR.