In some areas of the country, it may be necessary to use mothballs. These always leave a horrible odor behind, and it can be a nightmare trying to get rid of it. There are a few methods you can use to eliminate the odor without just covering it up.
Ionic and Ozone Products
Using an ionizer can remove the mothball smell quickly. They work by releasing particles that will attach to and pull in the molecules that cause the mothball smell. These are used often in hotels and other businesses where cleanliness is a must. You can find these at most big box retailers and online.
Place some small bowls of real charcoal (not the self starting charcoal sold for grills) around the area that has the smell. The charcoal will absorb the odor over time and can be tossed when it's done. It's important not to leave this around where children or pets can get a hold of them though.
If you are looking for a more natural way to clear the air, try a small bowl of fennel seed. Leave this in the area for a few days and the smell should dissipate drastically. Fennel seed is natural and has less risk involved to pets and children over the alternatives.