Castleford have lost their appeal against a £40,000 fine imposed after crowd trouble at a home game in June.
The Tigers were punished by the Rugby Football League after fans laid siege to the match officials' changing room following a loss to Catalans Dragons.
Objects, including bottles, were also thrown onto the pitch by spectators.
Half the fine was suspended until the end of 2010, and an RFL appeals panel gave the club six months to pay the £20,000 instead of the usual 28 days.
When handing out its punishment, the RFL's operations tribunal described the scenes on 27 June as "completely unacceptable".
Castleford, who were found to have breached the RFL's respect policy, were instructed not to allow spectators to take bottled beer on to the terraces in future.