Broncos managing director Bruno Cullen said the players were advised of their contract terminations following a management meeting this morning.
Neither Seymour nor Costigan will play for the Broncos again this season, or the Broncos feeder club the Toowoomba Clydesdales. Both players had Season 2007 to run on their contracts.
"This is a very sad day for this great club, and the decision we have made was not taken lightly," Mr Cullen said.
"However, enough is enough. As both an organisation and a sporting team, the Broncos is an integral part of the fabric of the Queensland community, and we do all that we can to fulfil our role within the community.
"At the Broncos we set high standards, and we expect those standards to be met. For that, I do not apologise. However, we also work with our players in various ways to prepare them to meet the challenges that we realise they will face."
Mr Cullen said that both the NRL and the Broncos operated under a strict code of conduct, and reiterated that players needed to realise the consequences if they stepped outside the boundaries that were stringently laid down. He said it was the responsibility of every player to be aware of the code of conduct, and the penalties for any breach.
"Today's decision should not be viewed as a knee-jerk reaction," he said.
"We have a duty to demonstrate to our players, our staff, our corporate partners and our fans that we will not stand for unacceptable behaviour" he said.
"I think it also important to point out that both Brett and Neville have been warned previously about their conduct, and I believe they should have been well aware of the consequences if they transgressed again. As a club, we can only tolerate so much."
Mr Cullen also announced today that the $5000 fine imposed on Neville Costigan yesterday, in relation to his drink-driving offence, would be donated to the Centre for Accident Research and Road Safety (CARRS-Q).
The Centre exists within the School of Psychology and Counselling, a division of the Faculty of Health at the Queensland University of Technology. It is located at QUTs Carseldine Campus, in Brisbane.