I asume this is a EPiServer CMS 5 website. If it is you can create your own replacement for the page "/systemui/edit/workspace.aspx" and maybe even the page "/systemui/edit/EditTree.aspx". Use a reflection tool and override the EPiServer class or create your own version of these pages where you disable the functionality for the Not Published List.

After this is done you have to make som additions to web.config.

First add the <virtualPathMappings> segment to the <episerver> section. Like this:

<virtualPathMappings>
   <add url="~/systemui/edit/workspace.aspx" mappedUrl="~/OwnUI/edit/workspace.aspx" />
   <add url="~/systemui/edit/edittree.aspx" mappedUrl="~/OwnUI/edit/edittree.aspx" />
</virtualPathMappings>

Secondly you must add (if it not already exist) a VirtualPathMappedProvider to the collection of VitrualPathProviders and place it as the last VPP, like this:

<add virtualName="ExtensionMapping" virtualPath="~/SystemUI/edit" bypassAccessCheck="false" showInFileManager="false" name="EPiServerUrlMappingVPP" type="EPiServer.Web.Hosting.VirtualPathMappedProvider,EPiServer" />

I hope this will help you.
/Peter