Sub Demo()
Application.ScreenUpdating = False
With ActiveDocument.Range
  With .Find
    .Wrap = wdFindStop
    .MatchWildcards = True
    .Text = "[! ^13]@[!.\!\?:;]^13"
    .Replacement.Text = ""
    .Execute
  End With
  Do While .Find.Found = True
    Select Case Trim(.Words.First)
      Case "and", "but", "or", "then"
        'do nothing
      Case Else
      .Words.First.InsertAfter ";"
    End Select
    .Collapse wdCollapseEnd
    .Find.Execute
  Loop
End With
Application.ScreenUpdating = True
End Sub