以下をフォームのコードに追加
' CreateParams プロパティをオーバーライドする Protected Overrides ReadOnly Property CreateParams() As System.Windows.Forms.CreateParams Get Const CS_NOCLOSE As Integer = &H200 Dim DcreateParams As System.Windows.Forms.CreateParams = MyBase.CreateParams DcreateParams .ClassStyle = DcreateParams .ClassStyle Or CS_NOCLOSE Return DcreateParams End Get End Property