The following should do that (for text watermarks like those mentioned).
Sub RemoveWaterMarks()
'Graham Mayor - http://www.gmayor.com - Last updated - 08 May 2018
Dim oSection As Section
Dim oHeader As HeaderFooter
Dim oRng As Range
Dim oShape As Shape
For Each oSection In ActiveDocument.Sections
For Each oHeader In oSection.Headers
If oHeader.Exists Then
Set oRng = oHeader.Range
oRng.End = oRng.Paragraphs(1).Range.End - 1
For Each oShape In oRng.ShapeRange
If oShape.Type = 15 Then oShape.Delete
Next oShape
End If
Next oHeader
Next oSection
lbl_Exit:
Set oSection = Nothing
Set oHeader = Nothing
Set oRng = Nothing
Set oShape = Nothing
Exit Sub
End Sub