Hi HG X HELLI, I'm sorry you're having problems with a certain user on Xbox LIVE. Please be aware however that the XBL Enforcement Team do not visit these forums, and don't contact individual users with harassment issues to investigate them. To have your issue investigated, you need to file a complaint on this person via your console.
The first thing you need to do is contact your ISP and tell them your connection has been attacked. They should issue your modem with a new IP address. Next you need to block communications with this user and change your online privacy settings, specifically, your online status. Once you change your Online Status to 'friends only', only Gamertags on your friends list will be able to see you as online. To everyone else, you will appear as 'offline', and the ability to join you will be blocked for them.
Please see the following guides:
Harassment and filing a complaint:
http://forums.xbox.com/xbox_forums/xbox_support/f/26/t/19139.aspx
How to change your Xbox Live Privacy settings:
http://support.xbox.com/xbox-live/how-to/online-safety
If you have any questions please post back. Thanks