Hey @DM2
First what @Gracjan has said is correct. To paint a little more context around that answer, when using the dnd_section tag your’re essentially creating a container or structure for modules and other structural elements. So by moving a module out of a section it will create another section and leave the empty section. You could use a single dnd_section like so:
{% dnd_area "home_body_dnd_area" label="Main section" %}
{% dnd_section %}
{% dnd_column
offset=0,
width=12
%}
{% dnd_row %}
{% dnd_module path="/theme-name/modules/Hero" %}{% end_dnd_module %}
{% end_dnd_row %}
{% dnd_row %}
{% dnd_module path="/theme-name/modules/CTA" %}{% end_dnd_module %}
{% end_dnd_row %}
{% end_dnd_column %}
{% end_dnd_section %}
{% end_dnd_area %}
Using the single section with a cilumn and rows will eliminate any additional structure being added to the layout. Though this will not stop the user from dragging modules into columns.
If you intention is to restrict users to the vertically stacked full width modules I might suggest a single module with a repeater that has a number of defaults that correlate to the modules you want them to use. This solution would allow them to add “modules”, rearrange them vertically, and restrict column creation. I will preface this with: It is possible but not recommended. I would instead implement a authoring process and a approval process.
If you have any questions please don’t hesitate to reach out!
Best,