Open your presentation
Click "Tools", "Macro", "Macros…" and type in removeNotes.
Paste the following into the window that appears, just before the text "End Sub"
Dim objSlide As Slide
Dim objShape As Shape
For Each objSlide In ActivePresentation.Slides
For Each objShape In objSlide.NotesPage.Shapes
If objShape.TextFrame.HasText Then
objShape.TextFrame.TextRange = ""
Close the script window.
Click "Tools", "Macro", "Macros…" and double click removeNotes.
Your notes should be deleted!
You can find another approach by Shyam Pillai over here
And if we started clearning up powerpoint…
Cool add-in is the Remove Hidden Data Add-in for Office 2003/XP
Then comes to your mind the question why its not build into the product the ability to remove the hidden data when saving… :-)