Removing Rafflecopter branding from the giveaway widget is available by upgrading to the premium subscription.
Normally, the widget would display a no-follow link on the bottom of the giveaway widget with an image of the Rafflecopter logo that points to the Rafflecopter homepage. After your promotion has run its course and the winners are chosen and displayed on the widget, we also advertise Rafflecopter as a service that everyone can use to run giveaways with.
With the premium subscription, this link / section of the widget is removed and the links displayed when the winners are announced are removed as well. Below is an image showing the differences between the widget with no branding (right) and the widget with Rafflecopter branding (left).