ok! I get it.
if a virtual folder "A" have 3 real subfolders x,y,z and virtual folder V. I select A and x and enable "no download", A and all subbfolder display "download forbidden". if I want to cancle "no download",just select A and x,uncheck "no download"
Acturally If you want give "no download" to the virtual folder A, just to select A and any real subfolder of A, you will enable "no download" to all real and virtual subfolder of A. Even the "any real subfolder of A" is a subfolder of V,it will be work.