Don't use moth balls; they are very toxic.

I've had the same problem with my wool sweaters, too. The only thing that has helped for me is to shake out my sweaters, or have them cleaned, and then put them in closed plastic containers that allow for some air. They shouldn't be airtight. I have also put cedar blocks inside with the sweaters. Although someone told me that cedar doesn't do anything, but I think that the smell repels the moths. Also, we have a chest of drawers with a large cedar-lined drawer in the bottom where we have placed wool clothing. I try to avoid buying wool anymore, unless I can keep it in a clothing bag or plastic box.