22
Add this to the UIElement/FramworkElement from where you do not wish the Alt+F4 to work.
wnd.KeyDown += new KeyEventHandler(wnd_KeyDown);
void wnd_KeyDown(object sender, KeyEventArgs e)
{
if ( e.Key == Key.System && e.SystemKey == Key.F4)
{
e.Handled = true;
}
}