The internal punishment follows the imposition last month by a Football Association-appointed independent regulatory commission of a 12-match ban and a ?75,000 ($117,000) fine.
The two fines combined means this latest episode of ill-discipline has cost the controversial player around ?500,000 (622,000 euros).
His club and the FA took action after Barton elbowed City's Carlos Tevez and then kicked out at Sergio Aguero as he left the pitch in the match on May 13, before aiming a headbutt in the direction of Vincent Kompany.
QPR stopped short of sacking Barton, but warned the player that his contract will be terminated should he "seriously breach the club's disciplinary procedures again".
They also banned him from their pre-season tour of Asia in July.
In a club statement, QPR chief executive Philip Beard defended the punishment, saying: "Joey's behaviour, which led to his red card at Manchester City, and his subsequent actions were completely unacceptable and we have made our feelings on this matter known to him.
"I am really pleased with the way in which the club and Joey have worked towards reaching a sensible conclusion and we fully believe he will respond to these sanctions in the correct manner."
Barton himself admitted that he deserved whatever punishment came his way.
"My behaviour was wrong and I accept the punishment that has been imposed upon me as a result," said Barton, who moved to Loftus Road on a four-year deal from Newcastle last summer.
"I apologise to the Manager, my team-mates and of course the QPR fans for my actions. I also apologise to the Manchester City players, staff and supporters."
In later posts on Twitter, the 29-year-old also insisted he will "bounce back."
"I was stupid and that's what happens when u do dumb things. I am confident ill bounce back. Cannot wait," Barton said.
"I wish to put it behind me now and help QPR, once available of course, push on towards the top half."
Barton added: "I was reckless and deserved every punishment I received. Money comes and goes, it's not important to me. Losing the captaincy and missing 12 games is what really hurts."
Barton has a long history of controversy, both on and off the pitch.
In 2008, he was given a six-month jail term after admitting assault and affray, while during his time as a player at Manchester City, Barton was fined after he stubbed a lit cigar in the eye of young team-mate Jamie Tandy during the club's Christmas party.
Earlier this month, Barton was arrested for affray following a night out in Liverpool with his girlfriend.
Source: AFP
Source: AFP