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At a top level (e.g. /Global, /Documents etc) I don't think its possible but its possible at a sub-folder level within the VPP folder using standard security. If you really need to you can modify the EPiServer UI or even remove the buttons as I blogged about here:
http://world.episerver.com/Blogs/David-Knipe/Dates/2010/5/Customising-the-EPiServer-UI/
David
Is it possible to somehow set a VirtualPathWssProvider VPP-directory as read-only so that the buttons for create folder and add file will be disabled?