Magpie Larks have very little response to sound devices.
The best way to deter them is by attacking their sense of sight.
Holographic bird scare ribbon flapping in the breeze works well on them.
The Indoor Laser also works well if undercover as it uses beams which resemble lightning bolts, which they are terrified of.
Taking away the reflection of the windows by making them opaque certainly helps as well.
If you have the room, the Scarybird Kite does a great job deterring all birds including Magpie Larks due to its ability to swoop and hover and the Prowler Owl is great for smaller areas such as decks or adjacent to pools and can conveniently be placed in a planter box.