You can hide the ribbon that displays above Nintex forms if you want to hide all the form ribbon options:
Please be aware that if you hide the ribbon for the form, you also hide the spell checking option and the ribbon options for the multi-line of text field as well:
Hiding the form ribbon
1. Open the form in the Nintex Form Designer
// Hide the form ribbon
4. Save the Settings and Publish the form.
Finished Example: The form will now open with the left navigation hidden: