In current version of Qt Creator it is not possible to print selected text (e.g. a single function in a file or whatever..).
A workaround is to use external tools together with a2ps, like described here : http://www.codegardening.com/2011/04/printing-from-qtcreator.html
However you will still miss code highlighting and the flexibility of a full printing framework, so I would like to suggest to add an option "print selected text" either in the printing dialogue or as a separate printing command (maybe on a right click contextual menu ??)