The 23-year-old has been linked with a move away from The Hawthorns and the Baggies have already turned down a third bid of £5million for the Scottish midfielder.
“We have said it many times, put statements out, and we’ll put another one out now. He’s not for sale,” Di Matteo told the Birmingham Mail.
“I’m not concerned about him though.
“We’ve made our position clear but you can’t force other clubs to not make these offers.
“Graham is a cool boy and he should be content in a club where he’s loved and settled.
“He has his first big year in the Premier League coming up.
“It would be so much better for him to stay here and continue his education. That reflects the view of everyone here.
“He’s an outstanding young player and there will always be talk about him.
“It makes sense for him to stay here.
“He’s got the respect of his team-mates and he has a management team which has belief in him – it’s important for any player to have that.
“There is nothing absolutely in his manner or mood which worries us. He is totally focused. He’s working hard and been very professional.”