Justified Season 6 has not been officially ordered by the cable network. The current Season 5 only debuted last week, with the next episode, The Kids aren't All Right making its premiere tonight, January 14.
John Landgraf made the following statement, saying that it was series creator Graham Yost and star/executive producer Timothy Olyphant who came to the decision to end the series next year.
"It was really Graham and Timothy Olyphant's decision [to end it]. We talked about it a year ago, and they felt that the arc of the show and what they had to say [fit better with six seasons than seven]. Regretfully, that's their decision."
The Season 5 premiere featured a touching tribute to author Elmore Leonard, who passed away last year at the age of 88, and whose characters the show is based on. Part of the decision to end the show so early came from his passing.