Disable access to external devices for specific users

[Raj Kiran Grandhi]

How does one disable access to external storage devices like usb flash 
drives and cdroms?

Not adding the users to the plugdev group does not seem to be working as 
the usb drives are automatically being mounted upon insertion.

Here is the output of `id guest`:
$ id guest
uid=1001(guest) gid=1001(guest) groups=1001(guest)


I am using Ubuntu 7.10 and this issue is present in both the 32bit 
version as well as the 64bit version.

-- 
Raj Kiran Grandhi