How to restrict user not to access any external device

Try this:
1. Download Devcon file at http://support.microsoft.com/kb/311272 and add it to your project folder.
2. Code:

using System.Diagnostics; 
...

//create Prcess start info object with devcon.exe 
ProcessStartInfo PSI = new ProcessStartInfo(@"C:/Devcon/i386/devcon.exe");

            
            // disable an USB CDROM 
            PSI.Arguments = @"disable *USB*CDROM*"; 
            Process.Start(PSI);

            //enable USB CDROM 
            PSI.Arguments = @"enable *USB*CDROM*";
            Process.Start(PSI);
...

// if you have specific devive ID or type such as Flash, Ipod, Samsung ....
// you can replace it in "CDROM" expression

It works without Administor permission 'cause .Net "trust" devcon.exe