How to remove this in MS Word?

Solution 1:

All you have to do is Google.

"Expand/Collapse is a feature built-in to all the default heading styles in Word except for No Space and Normal. There is no option to disable the Expand/Collapse feature unless you will be using the Normal style or you will be creating a custom style based on the Normal formatting."

How do you remove all instances of expand/collapse in Word 2016?

Solution 2:

That "arrow" appears because the style for that text is set to either Heading 1, Heading 2, etc. Clicking the arrow allows you to expand or collapse texts under that heading. To remove it, you must change the style to Normal or another style that is not for headings.