I wouldn’t use salt to kill them. It’s not known if snails can feel pain, and if they can, salt is probably a pretty nasty way to die. These snails can’t survive freezing, can they? I would put them in a container and put that container in the freezer.
Killing the snails is good, period. There’s no need to do anything with the shells. You could collect the snails at the park and then simply dump the shells into your yard.
Or you could sell art made from them, and put a disclaimer with the art that says this is from an invasive species.