I have a similar but slightly different question that was addressed in this post How do I create "headings" for section of checkboxes without them becoming field values? .
"headings" were created to break up the values listed on a form for the select field type "Program of Interest". Is there a way to make it so the headings won't be be a valid selection to choose? The first line of text works this way automatically, in my case "Select an Area of Interest:*". In the simple editor the first line has no stored value, in the advanced editor it disappears so I assume this is a built in function of the marketo form. When I remove the stored value for the headings and save, it automatically fills it back in with the display value.
I don't know if this further complicates things but I'm also using visibility rules. The "program of interest" is hidden until the "level of interest" is selected.