The most complete way to disable the optical drive is to remove the driver for the device (or move it to another location, so it can be restored if need be).
Try moving /System/Library/Extensions/IODVDStorageFamily.kext and /System/Library/Extensions/IOCDStorageFamily.kext to someplace outside Extensions. Then delete /System/Library/Extensions/Extensions.kextcache and reboot
(Note that if this doesn't work, you can restore the status quo ante by putting the two kexts back in their original directory, and running 'repair permissions' from Disk Utility)