It's simple really. Don't sign in for at least 31 days, and on the 32nd day, log in and viola you have a clean inbox and spambox.
I know this because it's been that long since I've check my hotmail account. Though I dont really use it, because I have about 3 gmail accounts I use. Only thing I get thats worth while in my hotmail account is the Beta testing stuff from microsoft, in which I might have to re-sign up for. But who knows.