[Try running `qlmanage -r`. This should reload QuickLook and make it so it's no longer focused on those files](http://superuser.com/questions/125841/how-can-i-free-files-that-are-in-use-by-quicklook). You can also try using something like [Trash It!](http://nonamescriptware.com/downloads/) if after restarting and relaunching/killing Finder and QuickLook does nothing.