They don't have a "cancel ballot" button. When you fill out and print the new online ballot it will cancel any previously sent ballots. They site says "Caution: If you continue, a ballot previously mailed to you will be canceled"
It wasn't an option, but it happens when you print out the new online ballot. The website has a caution notice about it when you go to print "Caution: If you continue, a ballot previously mailed to you will be canceled"