How can I get quicklook to ignore network volumes?
It is not quicklook that is trying to generate previews on its own, it is Finder that triggers it when you access that (and any other) folder on any mounted drive.

When in the folder, open Finder / View / Show View Options (shortcut Cmd+J), deselect Show Icon Preview and click Use as Defaults. That information is stored in the .DS_Store file in each folder.