Form2 f2 = new Form2();
                f2.ControlBox = false;
                f2.Show();

------解决方案--------------------
如果仅仅是关闭按钮变为灰色不可用，那么如下操作

C# code
[DllImport("USER32.DLL")]
            public static extern int GetSystemMenu(int hwnd, int bRevert);
            [DllImport("USER32.DLL")]
            public static extern int RemoveMenu(int hMenu, int nPosition, int wFlags);


            const int MF_REMOVE = 0x1000;


            const int SC_RESTORE = 0xF120; //还原 
            const int SC_MOVE = 0xF010; //移动 
            const int SC_SIZE = 0xF000; //大小 
            const int SC_MINIMIZE = 0xF020; //最小化 
            const int SC_MAXIMIZE = 0xF030; //最大化 
            const int SC_CLOSE = 0xF060; //关闭


            private void button1_Click(object sender, EventArgs e)
            {
                Form2 f2 = new Form2();
                int hMenu = GetSystemMenu(f2.Handle.ToInt32(), 0);
                RemoveMenu(hMenu, SC_CLOSE, MF_REMOVE);
                f2.Show();
            }

------解决方案--------------------
参考链接

------解决方案--------------------
将form2的FormBorderStyle设为none看，是不是你想要的