I need to customize the registration form to match our site.

For those with similar question the registration header text and image can both be adjusted in CB > Configuration > Registration.

I didn't realize that community builder would override our normal registration page.

This is explained in our documentation and shown on our demo site below.

demo.cbdemosites.com/

Also, is there way to go back to my CMS registration form?

No, CB requires its registration form or they won't exist in CB.

I found the configuration. Is there a way to remove the Join Us! ?

Use language overrides as shown below to remove it by translating it to an empty string.

www.joomlapolis.com/blog/kyle/18701-language-overrides-have-arrived

If I'm not mistaken, it looks like I'd need to purchase a captcha plugin?

Correct, you will need CB AntiSpam for Captcha.

I don't plan on doing this. So, how to I revert back to my CMS registration?

To do so would mean to not use CB.