Complex sentence: While these actions temporarily reduced global spam levels by around 75%, the effect did not last long: spam levels increased by 60% between January and June 2009,40% of which was attributed to the Rustock botnet.
Split into simpler sentences: While these actions temporarily reduced global spam levels by around 75%, the effect did not last long. Spam levels increased by 60% between January and June 2009,40% of which was credited to the Rustock botnet.