The quickest way to get a simple section heading is to add the [discrete] block style to it.
----
[discrete]
== Just a plain 'ol heading
----
However, I think that the backend should only be creating a section for level-1 headings. The rest of the headings (which would be slide content) should be treated normally (not as sections). It's necessary to check the level in the section template. I've filed an issue.