This frame has had a spinning bottle cage rivnut since coming here
@longun never got round to it but last night, after numerous previous attempts to remove it, I got serious. This is how it went.
Cutting disc in dremel. Cut off most but not all of head of stainless allen bolt stuck in nut. Change to blue grinding head in dremel. Grind off rest of head down to ally rivnut flange. With needle nosed mole grips compress and pull up flange with point of grips a little section at a time head on. - softer than head so clamp on and pull until it releases. Do this a lot for about ten minutes until you've created a little ally cup around the remaining bolt stump. The stump protects the frame for the next bit as the ally grinds more easily. Grind off the cup and repeat the process. Each time the cup edge gets thinner until eventually you can push the stump into the frame. If you're super careful with the grinder there's no damage to the frame.