This is controlled in the layout.
On the Layout tab (or from the Game window), edit the cell containing the prizes. Edit the HTML. Select some of the text of the <prizes> token and then press "Insert Token". This should open the "Insert Token" dialog, but have the "prizes" token already selected, with the appropriate attributes configured. Check the box next to "roundOff" and set it to "true".
If the Cell containing the <prizes> token has multiple Property Groups or Property Sets, make sure you make the same change for all of them.
Don't forget to save the layout when you're done.