Hi, unfortunately the Library Browser doesn't support multiple delete.
We have plans to rewrite this tool completely. For this reason we have not improved it lately.
For massive editing, we usually modify the "index.xml" file directly. It has an understable format, and can be editor with any text editor.
There is an index.xml by library. To locate it, use right click menu->"Open in Shell" with any library selected.
I hope that helps.