Do not use rubber for an exhaust gasket. It can't take the heat. While you're turning the nut can you see the stud (threaded part the nut is on) turning with the nut? If it is then the stud or the threads in the cylinder are stripped. If you own a pair of vice grips you might try pulling outward on the nut as you turn it. If all else fails a local mechanic, auto or motorcycle shop, might give you some help. Once the nuts/studs are out and if the threads are still good replace them with Allen head cap screws, available at Ace Hardware, They are 6mm diameter and about an inch long. Use gasket material made for exhaust systems, auto parts store item. You'll need to cut the gasket material to fit the exhaust flange. Use a hole or leather punch to make the holes for the bolts. Once everything is back together tighten the bolts just enough to flatten the lockwashers then just a little, very little, more. If in the worst case the threads in the cylinder are stripped they can be repaired with a device called a Heli-Coil thread repair kit. Installing them is not hard but requires a little understanding of the device and how it works. Good luck and get back to us if these suggestions don't help you.
Tom