It is possible, but it is usually not recommended. Microsoft decided not to include an API for that, so if you want to do it, you have to use methods that were not designed for that purpose.
One option could be to manipulate the back stack, removing all entries until the application exits.
Another option would to throw an exception and not handling it. This will cause your application to get killed.
In Windows Phone 8, though, an Application.Terminate() method was introduced, for those cases where there is really a need to terminate an application programmatically.