Sub KillAllBookmarks()
With ActiveDocument
  Do While .Bookmarks.Count > 0
     .Bookmarks(1).Delete
  Loop
End With
End Sub

Sub KillHiddenBookmarks()
Dim i As Long, bHid As Boolean
With ActiveDocument
  bHid = .Bookmarks.ShowHidden
  .Bookmarks.ShowHidden = True
  For i = .Bookmarks.Count To 1 Step -1
     With .Bookmarks(i)
      If Left(.Name, 1) = "_" Then .Delete
     End With
  Next
  .Bookmarks.ShowHidden = bHid
End With
End Sub