Via TVLine, which broke the story:
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In a surprising move, Starz is pulling the plug on Spartacus after the upcoming third season.  The final chapter, to be titled Spartacus: War of the Damned, will air in January 2013.

“Starz always does the unexpected,” Sparty’s creator, Steven S. DeKnight, told IGN. “This show has been challenged, to say the least, in many, many ways. We’ve faced many difficulties and tragedies on the show. My original plan was to attack the show in a five- to seven-season arc. Once we got into it and after Andy [Whitfield]’s passing and looking at the historical story of Spartacus, we came to the decision to basically end on a high note.

“We certainly could have continued for a few more seasons and stretched it out, but we really wanted to end high and not feel like we were treading water – and really just condense the rest of the history into one amazing ten-episode, badass final season,” he continued. “You know, the whole show was a huge risk from the start for Starz, and it was another huge risk when I decided to try to keep the show going after Andy’s passing. Any normal studio would have just cancelled the show at that point and not risk the financial downside of it not working. But Starz, thankfully, really stuck by the show, and they really wanted it to be told all the way to the end. They didn’t want to cheat the viewers and just suddenly pull the plug. So they gave us this opportunity to wrap up the story, and we’re certainly taking that opportunity and running with it. This is by far the biggest season we’ve ever attempted.”

War of the Damned will center on the final battle between Spartacus and Marcus Crassus. “It is very much Crassus along with the aid of Caesar going after Spartacus,” DeKnight previously told TVLine. “The scope of it multiplies many times as Crassus battles Spartacus across the country.”