Formun; üst menüsündeki kapatma tuşunu etkisiz hale getireceğiz.

Public Class Form1
    Public Class CloseButton
      Private Declare Function GetSystemMenu Lib "user32" (ByVal hwnd As Integer, ByVal revert As Integer) As Integer
      Private Declare Function EnableMenuItem Lib "user32" (ByVal menu As Integer, ByVal ideEnableItem As Integer, ByVal enable As Integer) As Integer

      Private Const SC_CLOSE As Integer = &HF060
      Private Const MF_BYCOMMAND As Integer = &H0
      Private Const MF_GRAYED As Integer = &H1
      Private Const MF_ENABLED As Integer = &H0

      Public Shared Sub Disable(ByVal form As System.Windows.Forms.Form)
      
        Select Case EnableMenuItem(GetSystemMenu(form.Handle.ToInt32, 0), SC_CLOSE, MF_BYCOMMAND Or MF_GRAYED)
          Case MF_ENABLED
          Case MF_GRAYED
          Case &HFFFFFFFF
            Throw New Exception("Kapatma menüsü bulunamıyor")
          Case Else
        End Select
      End Sub
    End Class
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
      CloseButton.Disable(Me)
    End Sub

    Private Sub Form1_SizeChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.SizeChanged
      CloseButton.Disable(Me)
    End Sub
End Class