The Wolves striker informed the Scottish Football Association via text message in February that he did not wish to be selected.
Now, Levein has said that the 24-year-old must use the same method of communication if he wishes to be considered again.
"He knows that if he wants to come back in, he just has to make me aware by the same means he bowed out," Levein said. "Nothing has changed. I haven't spoken to Steven.
"This is not about who looks bad or who doesn't. It's just that in any group there has to be rules for everybody. For me, it's much simpler if the rules are adhered to by everybody."
Fletcher did not feature in Wolves's 2-0 defeat away at West Bromwich Albion on Sunday.