using System.Runtime.InteropServices;

// …

[DllImport("user32.dll", EntryPoint="GetSystemMenu")]
private static extern IntPtr GetSystemMenu(IntPtr hwnd, int revert);

[DllImport("user32.dll", EntryPoint="GetMenuItemCount")]
private static extern int GetMenuItemCount(IntPtr hmenu);

[DllImport("user32.dll", EntryPoint="RemoveMenu")]
private static extern int RemoveMenu(IntPtr hmenu, int npos, int wflags);

[DllImport("user32.dll", EntryPoint="DrawMenuBar")]
private static extern int DrawMenuBar(IntPtr hwnd);

private const int MF_BYPOSITION = 0×0400;
private const int MF_DISABLED = 0×0002;

// …

IntPtr hmenu = GetSystemMenu(this.Handle, 0);
int cnt = GetMenuItemCount(hmenu);

// remove ‘close’ action
RemoveMenu(hmenu, cnt-1, MF_DISABLED | MF_BYPOSITION);

// remove extra menu line
RemoveMenu(hmenu, cnt-2, MF_DISABLED | MF_BYPOSITION);