Hi Gregory,
thanks for using MH Magazine lite.
I have set, then deleted featured image but the beige blocks remain.
You could hide the image placeholders with a line of CSS:
.mh-image-placeholder { display: none; }
You can add CSS code in your WordPress dashboard under Appearance => Customize => Additional CSS.
How can I eliminate the sidebar for existing and all future posts?
The free theme doesn’t have an option included to globally disable sidebars. This layout feature would be available in the premium version of MH Magazine, amongst various other additional features, options and custom widgets to configure your website.