AFAIK, there's no way to turn 'off' the behaviour.
You could use a macro like:
Code:
Sub KillAltText()
Application.ScreenUpdating = False
Dim Rng As Range, Shp As Shape, iShp As InlineShape
With ActiveDocument
For Each Rng In .StoryRanges
With Rng
For Each Shp In .ShapeRange
Shp.AlternativeText = ""
Next
For Each iShp In .InlineShapes
iShp.AlternativeText = ""
Next
End With
Next
End With
Application.ScreenUpdating = True
MsgBox "Finished updating", vbOKOnly
End Sub
For PC macro installation & usage instructions, see:
http://www.gmayor.com/installing_macro.htm
There is no Word setting to vary the default Alt Text behaviour.