都没人回答，自己纠结了1天终于弄好了，

        [DllImport("user32.dll")]
        public static extern IntPtr GetSystemMenu(IntPtr hWnd, bool bRevert);

        [DllImport("user32.dll")]
        public static extern bool DeleteMenu(IntPtr hMenu, uint uPosition, uint uFlags);

                IntPtr hMenu = GetSystemMenu(Process.GetCurrentProcess().MainWindowHandle, false);
                if (hMenu != IntPtr.Zero)
                {
                    DeleteMenu(hMenu, SC_MOVE, MF_BYCOMMAND); //删除移动菜单
                    DeleteMenu(hMenu, SC_MAXIMIZE, MF_BYCOMMAND);//删除最大化菜单
                    DeleteMenu(hMenu, SC_SIZE, MF_BYCOMMAND);//删除大小菜单
                }