There isn't really a good way to do it. There is a nice explanation/overview of your options here.
For short, if this is a Silverlight app (not XNA), it is not supported. You can simply throw an unhandled exception, and the app will quit. I wouldn't recommend that, it seems like a hack and a rather crude way of doing it.
Here is a way to make it look nicer, but at the end of the day it still throws an exception. I don't know if the application certification process looks at whether you are throwing unhandled exceptions, but I guess it could be an issue.
