Reprohuset,
I've looked around a bit, and after searching... I couldn't find any ways to disable it. Even looking at the various php files, I couldn't find what makes it appear.
So it looks like there is no ways to disable the discount voucher from showing up. This could be something to bring up to the developpers, however, and see if they may know a way, or if not, make it a feature request.
Regards,
Raphaël Lalonde Lefebvre