If this person has your IP address you should contact your Internet Service Provider. It is your ISP that assigns your modem/router with an IP address. They can change it for you, just tell them that someone has done a denial of service attack on your Internet connection.
Next be sure to file a complaint against this user. Microsoft takes a dim view of this kind of activity. Anyone found to be IP flooding other users will be subject to an account and a console ban.
As the above poster has already stated, your account is perfectly safe as long as you do not tell anyone your Windows LIVE ID and password. Also, only the Xbox LIVE Enforcement Team can ban a console from going online, so you don't need to worry about that.
It's not possible to IP ban a user, since all it would take to stop the IP block would be to reset their Internet equipment or contact their ISP for a new IP address.
It is not advisable for you to place your email address here as this is a public forum. I have asked a moderator to remove it for your own safety.