This is the same question of this link:
Removing [nid:n] in nodereference autocomplete
According with the first answer (Grayside) I've created my own module and activated. Then I create a new content, I look sth up in the nodereference field and finally select it -> it works (Doesn't appear the [nid:n]).
But, when I view/preview or save or edit the content, the [nid:n] appears again.
Anybody can help me?