How to prevent "question mark in block" characters co

This is partially an Excel question, but I'm writing a stylesheet to
convert an XML file to a CSV, and I have to do some things to make it a
valid CSV, like quoting the values, escaping quotes, etcetera.  One
thing that I don't see how to solve is that if the resulting CSV field
has embedded newlines, when I view the result in Excel, the end of each
embedded line has a "question mark in block" character.  How do I have
to massage the transformed xml data to prevent that?