If the rivnut is free to rotate in the frame then trying to drill it out is unlikely to lead to anything but massive frustration. But what you can do is to grind the topside flange off, preferably using something like a dremel. That will let the inside flange drop into the frame where it can either be shaken out through the BB, or you can squirt in a little rustproofing to keep it from rattling. Then just find someone who can install a new rivnut for you.