If you just want a plain text campaign without any HTML styling, you can choose the dropdown option next to the Compose button in your Dashboard. Then choose Just text.
This will open up the plain text editor so you can create a new campaign with no styling.
Keep in mind that without any HTML in your email, you cannot include images, styling, or clickable links in your email. And you won't get any statistics reporting on your mailing. All that requires HTML code, to function.