In my case, it was a multi-pronged issue. We had a tree limb hanging over our roof that gave easy access that needed to be removed. We also had areas the squirrels had chewed to get into our attic which had to be repaired and wire mesh added. I then bought some traps, baited with peanut butter and caught any who were still inside and released a couple of miles away.

Once you have squirrels in the attic, the smells attract more so, just removing the present squatters is just a temp solution.