How to disable alt+left click to drag windows?

[leszek]

Thats indeed an interesting problem. By default every window manager catches that to move around windows.
You can change that luckily in the window manager -> window behavior settings -> window actions.
If you don't find it execute

Code:

kcmshell5 kwinoptions

to get to it directly.
Under the window actions tab you can set up what happens on Alt+LeftClick. Setting it to nothing is probably what you want.