Here is how you leave the family plan
Sign into Xbox LIVE on your console.
Go to Settings, and select Account.
Select Leave Gold Family Pack.
If there is an error on leaving, then the parent account has to do it first then you can do it as instructed above.
On your console, sign in to Xbox LIVE with the primary account for the Gold Family Pack.
Go to Settings, and select Family.
Select the member you want to remove from the Gold Family Pack.
Select Remove From Gold Family Pack.
FAQ
support.xbox.com/.../family-pack-faq