Squirrels are a protected species, so the only thing labeled and legal for getting rid of squirrels is to trap them. You would do that using a havahart trap 0745. Once you have caught the squirrels in your attic you should release them over 10 miles away from your home. The only real way to prevent future squirrels from re-entering your home is to exclude them. You have to find where the squirrels are getting into the attic and close up the entry points. That is the only option for future squirrel free living.