We do not currently have any negative filters or searches in Paperpile (you can see more info on this thread), but there’s a workaround you can use.
A negative folder/label can be created by adding all papers to the folder and then going to the label you want to exclude, selecting all papers in there, and using the folder menu to remove the papers from the other folder. This will not stay updated when you add more papers, but may be useful in a pinch.