How long has the duckling had this you are calling it a wart? are you sure that is what it is? You can follow Eggcessives advice which is what I would do, But the only way your going to keep her from scratching it is to somehow cover which can be done but it will take some work to get a bandage right so she can eat and drink. A non stick bandage folded or cut to size after you treat then vet wrap to hold it in place you'll have to be careful with vet wrap that you get it snug enough she can't scratch it off but not snug enough it cuts off circulation. Getting someone to hold her wrapped in a towel would help keep her calm. But cleaning it up and taking pictures so we can see what it looks like without blood on it. Thanks.