
TIA
In your collection of tools do you have a Vise-Grip (also know as a locking pliers)? It's pretty much mandatory if you want to free up something like this.
When you put the new clamp on don't go crazy tightening it. You should only tighten it with as much pressure as 2 fingers can muster. If you've got to start gripping it with palms or both hands, that's starting to get too much.
Also you may have a situation called galling. This is very common with bolts and nuts made of soft stainless steel, which may be the case here. The threaded parts actually weld themselves solid from the friction of being threaded together. You should apply some grease or oil to the threads to prevent this. Just about any automotive lubricant will work.