There are two ways to remove the header and/or footer of your site. You can remove it from your entire site or per page.
Steps how to disable header of the entire pages of your site.
On the top left of your admin page click "Customize"
Next click Header
Then Header Layout
Under layout field select "None" from the dropdown menu. Hit Publish once you are done to save your changes.
Disabling Footer of all pages of a site. Go to Customize > Footer
Next click Footer Layout
Finally in the Layout field choose "None" from the dropdown menu and click Publish to save your changes.
On the other hand if you prefer to disable the header and footer on specific pages of your site follow the instructions below.
To remove it per page go to Edit Page then select the "No Header/Footer" in the template field under page attributes. Hit Publish when done.