The Engage Super League club were fined 40,000 after being found guilty of misconduct, although they have signalled their intention to appeal and have 15 days to do so. Sections of the Castleford crowd directed homophobic chanting at Crusaders winger Thomas during the League fixture between the two clubs at the Jungle on 26 March. Thomas, who announced he was gay in December, joined Crusaders in March and the Castleford game was his second for the club.
A statement from the RFL read: "Castleford were found guilty of unacceptable behaviour, of breaching the RFL's respect policy, of misconduct by their supporters and of conduct prejudicial to the interests of the sport.
"The tribunal was chaired by his Honour Judge Rodney Grant, who criticised the club for failing to take steps to stop the homophobic chanting, for failing to identify the perpetrators, for failing to challenge the chanting and for their failure to undertake a meaningful inquiry afterwards."
The Tigers, who must pay 20,000 immediately with the other half suspended until the end of 2011, have also been ordered to forfeit the 20,000 suspended from the 40,000 they were fined last year after bottles were thrown on to the pitch and officials were threatened following a game with Catalans.
Thomas' manager Emanuele Palladino issued a statement congratulating the RFL on their decisive action. "Today has been an historic day," he said. "Gareth Thomas is all about delivering positive messages and positive changes and, to that end, today was a huge step forward for Gareth and the world of sport. It is not about the fine, but rather recognition that this behaviour from a small section of the Castleford fans is not acceptable in 2010.
Unfortunately, what happened at Castleford could not be ignored. We are delighted that the RFL have taken action against what is a serious and upsetting incident for all parties."