The following code will do what you asked.
Code:
Sub Macro1()
Dim aTbl As Table
For Each aTbl In ActiveDocument.Tables
'aTbl.PreferredWidthType = wdPreferredWidthPercent
'aTbl.PreferredWidth = 100
If aTbl.Columns.Count > 1 Then aTbl.Range.Cells.DistributeWidth
Next aTbl
End Sub
The two disabled lines could be enabled if you want all the tables to fill the width of the page as well.