The only truly effective way to remove pills from a garment is to shave it. I would recommend using the cheapest disposable razor you can find and simply shave the entire garment. It is time-consuming, but effective. Depending on the size of the article, you may need more than one razor to do a good job. You can also try using a pumice stone, or a commercial product called a Sweater Stone that is made specifically for this purpose.