How do I comment out a block of tags in XML?

You can use that style of comment across multiple lines (which exists also in HTML)

<detail>
    <band height="20">
    <!--
      Hello,
         I am a multi-line XML comment
         <staticText>
            <reportElement x="180" y="0" width="200" height="20"/>
            <text><![CDATA[Hello World!]]></text>
          </staticText>
      -->
     </band>
</detail>

You can wrap the text with a non-existing processing-instruction, e.g.:

<detail>
<?ignore
  <band height="20">
    <staticText>
      <reportElement x="180" y="0" width="200" height="20"/>
      <text><![CDATA[Hello World!]]></text>
    </staticText>
  </band>
?>
</detail>

Nested processing instructions are not allowed and '?>' ends the processing instruction (see http://www.w3.org/TR/REC-xml/#sec-pi)

If you ask, because you got errors with the <!-- --> syntax, it's most likely the CDATA section (and there the ]]> part), that then lies in the middle of the comment. It should not make a difference, but ideal and real world can be quite a bit apart, sometimes (especially when it comes to XML processing).

Try to change the ]]>, too:

  <!--detail>
    <band height="20">
      <staticText>
        <reportElement x="180" y="0" width="200" height="20"/>
        <text><![CDATA[Hello World!]--><!--]></text>
      </staticText>
    </band>
  </detail-->

Another thing, that comes to mind: If the content of your XML somewhere contains two hyphens, the comment immediately ends there:

<!-- <a> This is strange -- but true!</a> -->
--------------------------^ comment ends here

That's quite a common pitfall. It's inherited from the way SGML handles comments. (Read the XML spec on this topic)