How to hide only the Close (x) button?

We can hide close button on form by setting this.ControlBox=false;

Note that this hides all of those sizing buttons. Not just the X. In some cases that may be fine.

Well, you can hide it, by removing the entire system menu:

private const int WS_SYSMENU = 0x80000;
protected override CreateParams CreateParams
{
    get
    {
        CreateParams cp = base.CreateParams;
        cp.Style &= ~WS_SYSMENU;
        return cp;
    }
}

Of course, doing so removes the minimize and maximize buttons.

If you keep the system menu but remove the close item then the close button remains but is disabled.

The final alternative is to paint the non-client area yourself. That's pretty hard to get right.

If you really want to hide it, as in "not visible", then you will probably have to create a borderless form and draw the caption components yourself. VisualStyles library has the Windows Elements available. You would also have to add back in the functionality of re-sizing the form or moving the form by grabbing the caption bar. Not to mention the system menu in the corner.

In most cases, it's hard to justify having the "close" button not available, especially when you want a modal form with minimizing capabilities. Minimizing a modal form really makes no sense.

You can't hide it, but you can disable it by overriding the CreateParams property of the form.

private const int CP_NOCLOSE_BUTTON = 0x200;
protected override CreateParams CreateParams
{
    get
    {
       CreateParams myCp = base.CreateParams;
       myCp.ClassStyle = myCp.ClassStyle | CP_NOCLOSE_BUTTON ;
       return myCp;
    }
}

Source: http://www.codeproject.com/KB/cs/DisableClose.aspx