The 35-year-old forward played more than eight of his 12 NLL seasons with the Mammoth, but has not suited up this preseason given the constraints of the new collective bargaining agreement. Prior to training camp, Prout was given the opportunity to sign a free-agent contract with any team in the league.
"For years, Gavin Prout was synonymous with the Colorado Mammoth," said president and general manager Steve Govett. "Unfortunately, he is a casualty of the reduced roster size and newly implemented salary cap of the CBA. Elements of the agreement made for some incredibly difficult decisions; this was the toughest."
"With just 16 runners dressing each game, there's a significant emphasis on speed and athleticism. Younger and faster players are the new NLL."
Prout is the team's all-time leader in assists (459) and points (664). In 2006, he captained the Mammoth to the league title, known as the Champion's Cup.
"I have nothing but great things to say about Gavin," Govett added. "We allowed him to field offers from other teams, and we're now forced to make this announcement as our 20-man roster is due to the league today.
"Our goal is to honor Gavin's illustrious career, and we intend to do just that when he ultimately decides to retire."
Also the Mammoth have placed forward Casey Powell on the team's holdout list.
Powell, a longtime star in the league who helped lead the Chesapeake Bayhawks to the Major League Lacrosse title this summer, said in a statement: "Due to recent circumstances in my personal affairs, I am unable to play for the Colorado Mammoth at this time. It is my sincerest hope that I will be able to return to the team in the future and help the organization win the Champion's Cup. I appreciate the support of the franchise, my coaches and teammates, as well as the great fans of Colorado, and can't wait until I'm able to get back to play again at Pepsi Center."