There are various ways by which you can disable a USB mass storage device. Here is how you can disable a USB device by editing grub.conf or menu.lst. Allyou need to do is open the file in a text editor and append nousb as one of the kernel parameters:
kernel /vmlinuz-2.6.18-128.1.1.e15 ro root=LABEL=/
console=tty0 console=ttys2,19200n8 nousb
Save and close the file and reboot the system to make the change effective:
# reboot