Log in to your account and click on the "Your Account & Help" option.
In the "Account Information" section at the top of the page, click "Cancel Membership," read the terms and accept them, then click "Complete Cancellation."
Sign up for a new Netflix account but use a new email address, username and password. If for some reason you need or want to use the same email address, call the company after canceling and request to have your email address disabled.
Keep your new Netflix password to yourself and log out after every viewing in order to keep your viewing history private.