how to delete the *.ismv files in the smooth streaming

I create a directory which contains the *.ism , *.ismc,*ismv files in the inetpub/wwwroot/ .  then the client requests the smooth streaming.

After that , when I want to delete the directory , I only can delete the *.ism,*ismc files . the *.ismv files can't be deleted unless I restart the IIS(seems the files is in use).  how can I delete these files without restart the IIS?  thanks a lot!