He only featured on the substitutes bench while he was at Sincil Bank - without making a single appearance, and the arrival of Aston Villa midfielder Paul Green and Glasgow Rangers attacker Dany N'Guessan have made him surplus to requirements.
"After careful consideration and having had a good look at Richard, we have decided not to pursue our interest in him," head coach John Schofield told the club's official website.
"This now gives him a chance to further his career elsewhere and we wish him all the best for the future."
Kell left Glanford Park at the end of the 2004/05 season to join Barnsley on a Bosman transfer.
But his exit was marred by a broken leg, which he suffered in the penultimate game of United's season - but after he'd agreed a move to Oakwell.
His time at the Tykes has seen him struggle to overcome that injury, and he has failed to break into the first-team, and he has been trying to find a new club over the last couple of months.