If you don't have a margin account, you can stop short sellers from using your stock to short with. I think that's very important and could take some of the volatility on the down side out of the market for SIRI. IMPORTANT, call your broker and tell him that you don't want your stocks to be shorted. Most people don't know about this right that they have.
You and the other poster are full of it. If you don't want your shares shorted, you have to take position of the stock. That means have your shares sent to you, and it will cost a few dollers to do this.