The Robins signed the centre-back in January, after his contract was cancelled at Glanford Park, but have decided he is too injury prone to be offered a new contract.
Gary Johnson, the Robins manager, revealed the decision was made after he failed an end-of-season medical.
"After Izzy's medical and discussions with my medical staff we felt it was best he moved on," he told the club's official website.
"That's nothing against Izzy, he was unlucky but we couldn't guarantee he would be available for the whole season.