The 24-year-old has been in good goalscoring form this season, leading to speculation that other clubs may bid for his services.
However, McCarthy has told potential suitors that they would be wasting their time by submitting an offer.
"There is not a chance of Fletch leaving. If people call, I'd tell them no. He's too valuable for us," said the Wolves boss.
Fletcher joined Wolves in 2010 from Championship club Burnley.