<p>The IntelliJ IDEA short-cut to reformat source code is <kbd>Ctrl</kbd>+<kbd>Alt</kbd>+<kbd>L</kbd>.  This happens to lock the screen in KDE (Gnome too?).  How can I disable this so IntelliJ receives the key sequence?</p>

Under the K menu, go to the Control Center.
Regional & Accessibility |
Keyboard Shortcuts |
Shortcut Schemes tab,
Global Shortcuts tab.
In the search box, type "lock", which should narrow the visible shortcuts and show the
Ctrl
+
Alt
+
L
one ("Lock Session").
Underneath, click None.
Click the Apply button.
On KDE Plasma 5 go to System Settings | Desktop Behaviour | Screen Locking
Here you will find a "Keyboard Shortcut" option. Change it to something else or set to none