its in
tpl_create_account_default.php
tpl_login_defalt.php

However if you turn it OFF where Kim ( ZC Founder ) stated the checkbox for the newsletter goes away

The Text HTML buttons are for ALL emails,

so rather than taking out the entire box, just change the verbage,
includes/languages/english/YOUR TEMPLATE/create_accont.php and login.php

make it say email preferences rather tha newsletter/email settings