Ciao, una soluzione veloce:
codice:
Dim isKeyPressOk As Boolean = True

Private Sub TextBox1_KeyDown(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles TextBox1.KeyDown
    If Not isKeyPressOk Then
        e.SuppressKeyPress = True
    Else
        isKeyPressOk = False
    End If
End Sub
Private Sub TextBox1_KeyUp(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles TextBox1.KeyUp
    isKeyPressOk = True
End Sub
Attenzione, che bisogna prendere in considerazione i tasti ctrl, shift, alt...