In that case you need to examine every paragraph with a loop
Code:
Sub WipeLocalFormats()
Dim aPara As Paragraph
For Each aPara In ActiveDocument.Paragraphs
With aPara.Range.Font
If .Bold = wdUndefined Or .Italic = wdUndefined Then .Reset
End With
Next aPara
End Sub
This assumes the underlying paragraph style on the 'mixed' paragraphs is non-bold and non-italic.