Remove "Yes. I want to receive updates via email." on Registration Form

Pro Form

If you are using pro form then you can use this code to hide it:

div.s2member-pro-paypal-form-submission-section>div.s2member-pro-paypal-form-custom-reg-field-opt-in-div {display:none;}

Add the code to the LiveEditor of the page >> Page Settings >> Other Scripts >> Custom CSS.

Default WordPress Registration Form

For default WordPress registration page, use this code:

<style> 
form#registerform p:nth-child(7) {
display: none !important;
}
</style>

Note that if you have added custom fields to the registration form, you'll have to change p:nth-child(7) accordingly. Increase 7 depending on the fields that you've added

Add the code to OptimizeMember >> General Options >> Login Page & Registration Form Design >> Login/Registration Footer Design.