Romano Fenati has apologised for what he called a "disgraceful gesture" after grabbing a rival's brake lever while travelling at 135mph during a race.
The Italian, 22, was banned for two races after the incident during the San Marino Moto2 on Sunday.
The Marinelli Snipers Team has since cancelled his contract and he has lost his ride at Forward Racing next season.
"I apologise to the whole sports world. This morning, with a clear mind, I wish it had been just a bad dream," he said.
"I think back to those moments, I made a disgraceful gesture, I was not a man.
"The criticisms are correct and I understand the resentment towards me. I want to apologise to everyone who believed in me and all those who felt hurt by my actions."
Fenati was disqualified after 23 laps for "irresponsible riding" when he grabbed the brake lever of compatriot Stefano Manzi, who briefly lost his balance before regaining control.
Fenati's actions followed Manzi's attempted overtake a few laps earlier. The pair had made contact as Manzi overtook on the inside, with both riders running off the track, losing several positions and dropping out of the points.
"We had a contact two turns before, but nothing to justify such a reaction. His gesture speaks for itself," said Manzi.
After cancelling his contract, the Marinelli Snipers Team called the incident "an irresponsible act [that] endangered the life of another rider".
Forward Racing, who Fenati was set to ride for next season, said in a joint statement with manufacturer MV Agusta that "Fenati's behaviour was incompatible with [their] sporting values".
'I've never seen behaviour as dangerous'
Giovanni Castiglioni, president of MV Agusta, said: "In all my years of watching sport, I have never seen behaviour as dangerous as this. A rider who can act like this can never represent the values of our company, and our brand."
Forward Racing owner Giovanni Cuzari said: "Our sport is already extremely dangerous, and any act which increases the risks involved for the riders is intolerable. We cannot accept behaviour of this type from one of our future riders."
Fenati added: "Unfortunately I have an impulsive character, but my intention was certainly not to hurt a driver like me but I wanted to make him understand that what he was doing was dangerous.
"I do not want to justify myself, I know that my gesture is not justifiable, I just want to apologise to everyone."
British rider Cal Crutchlow, who finished third in the main MotoGP race later on Sunday, said Fenati "should never compete on a motorcycle again".
He added: "He should have walked back to the garage and his team should have just kicked him straight out the back.
"You can't do this to another motorcycle racer. We are risking our lives enough."
In the warm-up of the 2015 Moto3 Argentine Grand Prix, Fenati was forced to apologise for kicking out at Finnish rider Niklas Ajo and he was dropped for disciplinary reasons during the 2016 season by the Sky Racing Team VR46, owned by Italian motorcycling star Valentino Rossi.