How can I disable 'Alt' + mouse default behavior in KDE?
Solution 1:
In the KDE System Settings for Window Behavior, change the Window Actions for Inner Window, Titlebar, & Frame. With the modifier key set to Alt, set the left button to Nothing. You can also reassign the action to a different modifier key.
See also: https://bbs.archlinux.org/viewtopic.php?id=109085
Solution 2:
although Bradd's answer is correct, if you want to disable alt for just one application/window, you can go to window behaviour -> <application> -> appearances & fixes and check "Ignore global shortcuts" there.