Agree most barcode readers work as keyboards (often they are plugged in between the keyboard and the system unit).
However you could be a little bit creative and embed additional characters in your barcode to acts as a validation string. for that to work you will have to make sure that their are no other barcode readers showing the data working in or around the site.
always intercept the keyboard input coming in.. if it doesn't include the additional validation characteristics then reject it or leave it on the screen as a come on to frustrate the fraudsters... after all knowing who is doing the fiddle is often a necessary step for stopping the fraud.
There are other things you could consider such as storing the data in a hidden control.. but cloning it to a visible control. allowing keyboard input to appear int the visible control.
I'd rather be riding on the Tiger 800 or the Norton