VB2005中如何禁用窗体右上角的关闭按钮？

    Public Sub frmMain_FormClosing(ByVal sender As Object, ByVal e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
        '退出系统
        Dim tw As Integer
        On Error Resume Next       

        tw = MsgBox("是否退出系统？", vbQuestion + vbYesNo)
        If tw = vbNo Then
            e.Cancel = True
            Exit Sub
        End If
        End

    End Sub