Brennan was charged with misconduct after entering a seating area during Toulouse's 28-13 European Cup victory over Ulster in January and punching a supporter.
The hearing imposed the strongest sanctions available, issuing a lifetime suspension from the sport and ordering a lifetime ban on participation in any capacity in tournaments organised by ERC.
Brennan was also fined 25,000 euros (Â£17,125), told to pay 5,000 euros (Â£3,425) compensation to Patrick Bamford, the fan he struck, and the costs of convening the hearing.
The 33-year-old had pre-empted the disciplinary hearing by announcing his retirement from the sport on Monday, but Friday's sanctions were still a clear indication of the severity of the incident.
An ERC statement read: "It was the view of the committee that Mr Brennan's behaviour was completely unjustified and that he caused serious harm to an innocent spectator and significant damage to the image of rugby union.
"The committee could not envisage a more serious misconduct in relation to spectators and believed that the maximum permissible suspension was appropriate."
Brennan had indicated before the incident that he would retire at the end of the season, although it is understood he was also looking at the possibility of playing for one more year.
His astonishing assault on Bamford ended any hopes of continuing, however, and although he trumped the hearing by calling time on his career at the start of the week, it is a bitter way for a respected competitor to exit the game.
The ERC statement continued: "The committee, having heard testimony from a number of witnesses and reviewed documentary evidence, found Mr Brennan guilty of misconduct under the Heineken Cup Disciplinary Rules in that he entered one of the seating areas in the stadium and repeatedly struck an Ulster spectator, namely Mr Patrick Bamford."
Brennan had been suspended by the French Rugby Federation until Thursday after the International Rugby Board requested Toulouse adopt the measure "in the interests of the game".
Bamford, 25, required medical treatment at the ground and on his return home to London and has since begun criminal and civil proceedings.
Brennan claimed he was provoked by derogatory chants during the game.