The Championship club say Griffin, 36, breached his contract by making three unauthorised appearances for Morley Rugby Union Club.
Griffin, who has also played for Wakefield Trinity, Huddersfield Giants and Salford Red Devils, was suspended in December pending an internal investigation into alleged gross misconduct, though news of his suspension was only made public last week.
Rovers say the front-rower was “categorically” told not to play rugby union during negotiations for a new one-year contract which he signed last August.
The club say that was written into his new deal and playing for a rugby union side without written permission from Rovers’ chairman or chief executive, as per the Rugby Football League (RFL) players’ contract, was an act of serious insubordination and an example of gross misconduct.
In a statement, Featherstone general manager Davide Longo said: “A player’s contract is designed to protect both the player and the club and any breach of it, on either side, has to be taken very seriously.
“We have a zero-tolerance policy when it comes to gross misconduct. Without this, you set a dangerous precedent.
“We are undoubtedly disappointed by Darrell’s decision to play another contact sport while under contract with the club.
“RFL players’ contracts clearly state that permission to do so must be granted, in writing, by a chief executive or chairman.
“Darrell did not seek such permission and, given the club’s position and desire to achieve as much as possible in 2018, it would not have been granted under any circumstances.”
Longo added: “At the time of agreeing Darrell’s new contract, we made it extremely clear that he was not to play rugby union. This condition was added to his written contract, which Darrell signed in August 2017.”
Griffin appealed after an initial hearing in December. The decision to dismiss the former England international was upheld at a second hearing on January 5 and Griffin was informed of the outcome on Sunday.