Is it the actual Diamond Shield product, or are you just using that as a generic name for some sort of clear paint covering? Note that "Diamond Shield" is a brand, and different than a similar product that 3M sells called "Scotchguard Paint Protection Film", or the 3M Paint Defender spay film. And all of those are different than the clear coat top-layer of the paint.
Frankly, I doubt if there is much that will help, but you could try something like Magic Eraser (use sparingly to see if it works without further damage).
Diamond Shield doesn't have much helpful suggestions either:
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL