Adriano Dhou -
Thank you for posting on the Xbox Video Community.
There's an option on the Xbox website to do this.
Sign in here, and scroll down. In the middle column, click on "Remove my profile from all Xbox 360 consoles" and follow the page from there.
Personally, I also recommend changing the password and making sure that your security proofs are up to date here:
http://account.live.com/proofs
Let us know if there's anything else you need help with!
Regards,
Kevin J S