Barton stripped of QPR captaincy, fined
Barton saw red in the final match of the season against Manchester City, which decided the English title and could have seen QPR relegated.
Barton elbowed City forward Carlos Tevez in the face and then kicked opposition striker Sergio Aguero while leaving the pitch before being escorted away by stewards.
The incident earned him a 12-game Premier League ban for violent conduct and a fine of $94,000 from the league.
"The Club has also reached agreement with Barton that if he seriously breaches the Club's disciplinary procedures again, the Club reserves the right to terminate his contract," QPR said in a website statement.
"I was reckless and deserved every punishment I received," Barton said on Twitter.
"Money comes and goes, it's not important to me. Losing the captaincy and missing 12 games is what really hurts. I was stupid and that's what happens when you do dumb things."
Barton is confident he will "bounce back" and help QPR "push on towards the top half", he added.
The 32-year-old joined QPR as a free agent in summer 2011, signing a four-year deal on a free transfer to become the club's highest paid player. He has scored three goals in 31 games since then.
Barton was released by Newcastle United after fiercely criticising the club's board and transfer policy on Twitter, and has been twice convicted of assault, serving a six-month prison sentence on one occasion.
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