Target.Font.Name = "Marlett"
 If Target = vbNullString Then
Target = "a"
Else
Target = vbNullString
End If

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    If Target.Cells.Count > 1 Then Exit Sub
    r = Cells(Rows.Count, 2).End(xlUp).Row
        If Not Intersect(Target, Range("A2:A" & r)) Is Nothing Then
Вот_зачем
'            Target.Font.Name = "Marlett"
'            If Target = vbNullString Then
'                Target = "a"
'            Else
'               Target = vbNullString
'            End If
        End If
End Sub

'Снимаем флажок, если был двойной щелчок по ячейке
Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
    r = Cells(Rows.Count, 2).End(xlUp).Row
    If Not Intersect(Target, Range("A2:A" & r)) Is Nothing Then
        Cancel = True 'чтобы не уйти в режим редактирования
Вот_зачем
'        Target.Font.Name = "Marlett"
'            If Target = vbNullString Then
'                Target = "a"
'            Else
'                Target = vbNullString
'            End If
    End If
End Sub

Sub Вот_зачем()
ActiveCell.Font.Name = "Marlett"
If ActiveCell = vbNullString Then
ActiveCell = "a"
Else
ActiveCell = vbNullString
End If
End Sub