It has been reported Birmingham have improved on their estimated £8million approach for the winger, who was Wigan's player of the season last term.
But Martinez has reiterated his desire to keep hold of the 23-year-old former Newcastle player.
He said: "My view is that we've always said that Charles N'Zogbia is not for sale and that's not going to change.
"I spoke with Charles, he's very happy, that's what matters - that's a clear message to our fans.
"We're not here to try and sell our best performers, we want to keep them, develop them and help them reach the next level and Charles feels that he can do that at Wigan Athletic.
"It's for everyone to see the huge development of Charlie's game over the last season, he has enjoyed his football and his role at Wigan and we want that to continue for next season."
Martinez added: "We are not in the position where we have to sell and if and when we do it will only be in the club's best interests and after we've identified two or three potential replacements for that position.
"We want to grow as a club over the next few seasons, we have a clear strategy for the way ahead and we are determined to keep progressing.
"The sort of speculation we have seen this week is, in my view, just malicious and designed to unsettle a player who is very happy here."