Private Declare Function SendMessageLong Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, _
             ByVal wParam As Long, ByVal lParam As Long) As Long

Private Sub Form_Load()
    List1.AddItem "Texto 1"
    List1.AddItem "Texto 2"
    List1.AddItem "Texto 3"
    List1.AddItem "Texto 4"
    List1.AddItem "Texto 5"
    List1.AddItem "Texto 6"
    List1.AddItem "Texto 7"
End Sub

Private Sub Command1_Click()
    'Seleccion de todo el contenido
    Dim Resp As Long
    Resp = SendMessageLong(List1.hwnd, &H185&, True, -1)
End Sub

Private Sub Command2_Click()
    'Eliminacion de todos los elementos seleccionados
    Dim Resp As Long
    Resp = SendMessageLong(List1.hwnd, &H185&, False, -1)
End Sub