Cherries dismissed the goalkeeper yesterday following what the League One outfit claimed was a further incident of misconduct.
Mitchell confirmed Stewart was no longer with the club and revealed the decision came after an issue last week, but he did not wish to go into detail about the alleged disciplinary matter.
Stewart joined Cherries in September and last month served a two-week club suspension for alleged breaches of discipline and a poor attitude to training.
The former Weymouth keeper returned to Cherries in time to make his fourth start of the season against Charlton Athletic, and subsequently insisted he was keen to show what he could do.
But speaking to the Echo yesterday, Mitchell said: I don't really want to comment on what the issue was, but there was an issue last week that we just could not tolerate at the club and we were not going to.
It was a behaviour matter and it was something we could not tolerate at the club. It was just a breaking of the rules and we could not be doing with it.
Stewart declined to comment yesterday morning and asked to be contacted later in the day.
Stewart was later unavailable for comment.
Mitchell added: It is disappointing because we feel it is a waste of talent, but we have to abide by the rules and it would appear he has broken them more than once.
We have got a great bunch of lads and we are not going to change that. We must abide by the rules or it does not work for us. It has not worked for Jon in this instance.
It is a disappointment in all walks of life to have to dismiss a young man, especially somebody who could perhaps have flourished with the club.
But people have to learn by their mistakes and it looks like that is going to have to be the way with Jon, I'm afraid. It is sad but we have got to look forward, move on, get somebody in to cover and keep on going.
Back-up goalkeeper Dan Thomas, who recently joined Dorchester Town on loan, has been recalled to Lee Bradbury's squad as cover for Shwan Jalal. Cherries retained a 24-hour recall clause on the homegrown goalkeeper.