A partition editor (PartedMagic, Gparted, (maybe) Partition Magic (not sure)) will allow you to mark the partition as write protected (partition flags), this should protect against formating and file deletion, however a user who knows what He/She is doing will still be able to clear the write protect.
Umm couple of quick questions.
1. Is it formating when you plug it in(with out command)
2. If it is have you tryed a diffrent usb port
3. Why are you trying to keep it from being formated.
A partition editor (PartedMagic, Gparted, (maybe) Partition Magic (not sure)) will allow you to mark the partition as write protected (partition flags), this should protect against formating and file deletion, however a user who knows what He/She is doing will still be able to clear the write protect.