Thanks for the request, @kahikatea, and welcome to our forum! We do not currently support negative filters / searches in Paperpile, but we will soon - this and a couple other advanced search features are currently being tested on stage.
In the meantime, 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.