I have a horizontal placed 16:10 monitor and a vertically placed 16:9 monitor.

In KDE, if I vertically maximized a window and drag it to the other monitor, it will automatically become vertically maximized on the target monitor.
And in my case, this is not reasonable to vertically maximized a windows on a vertically placed 16:9 monitor.

In some previous version of KDE I've succeed to find some place in system settings to disable this function.
But I forgot where to disable that function, and I cannot find it any more in KDE 4.9.3.

So would you please tell me how to disable this?

I'm thinking it's a bug, as I experience this often. I think I even reported it, but I can't find the bug number at the moment.

Does it still do that if you enable "Display borders on maximized windows" in System Settings -> Window Behavior -> Window Behavior -> Moving?

There's no way to deactivate that behavior and it's not a bug. (There has been a bug that broke partial maximization accidentally on move events what would as a side effect have prevented such resizes on screen geometry changes)

The reason is that maximizing a window is (unlike the zoom button, ie. the green marble in OSX) not a shortcut to get the a specific size, but a hint to "give it as much of the screen area as possible" (netwm spec)

What you can do is to either restrict the maxium size (height) of random windows though the rules (the window is still maximized but won't of course grow beyond the permitted size) or the workspace area of the screen through struts (what docks do, but you'll probably artificial strut) or simply unmaximizie the window.

inb4:
i'd very much prefer a "zoom" system above a "maximizing" one myself and i *think* so does the kwin maintainer, but the "ZoomedState" was withdrawn from ICCCM long ago and there's simply no protocol for this in place atm. and it's not possible w/o supporting hints from the clients.

OK... Thank you all for your kind messages.

I think should just drag the border to do vertical maximization instead of middle click on maximize button.
This will prevent the problem.

Thank you.