Data regions (matrix, table, list, etc.), groupings (except chart groups
where Labels have a different meaning), and textboxes have a property called
Label. If the Label property is set on any of these and the expression
returns a value != null, the document map will show up.

You can open an RDL file in a XML editor, report designer code view, or
notepad and just search for <Label> elements and remove them if you don't
want them. However, make sure to *not* remove <Label> elements on report
parameters or on chart groupings (because these are the labels shown inside
the chart).