I'm trying to picture what you are writing about, but I am a bit confused. If you are talking about the rear bolt on the lower control arm, then that is just connected to the control arm itself. Just remove the front nut and bolt and the ball joint and take the whole control are out and replace it. A picture might help with your description.
OK, I reread your post and it seems that you are talking about the clamp itself. IF so, then CMill's advise is good. If there is not enough of the bolt sticking out, then you will need to drill it out and possibly use an extractor.
OK, I reread your post and it seems that you are talking about the clamp itself. IF so, then CMill's advise is good. If there is not enough of the bolt sticking out, then you will need to drill it out and possibly use an extractor.