Download this utility to make your host computer unable to write anything on USB drive:
Download USB Write Protector
Or else goto properties by right click on your USB drive> Security > [ Uncheck ] / or whatever OS you are using, Write only option. You are done.
This will at least, will not allow anyone to write on your USB, drive.
" I should mention the simplest solution of all: just buy a usb key with a physical read-only/read-write toggle switch. These used to be pretty standard, though they're a little harder to find these days, it's certainly not impossible to track them down. "
I would like to see how it works. Because, if (the USB) works as a tiny hard drive, I'd prefer to use it only as a Read Only memory.
[ARE YOU BORED, DON'T BE ;]