How to suppress unnecessary xml namespace prefix declaration?

I receive this xml

<message xmlns="http://www.ns1.com" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <elem1 id="att1">
    <elem2 i:nil="true" xmlns:i="http://www.w3.org/2001/XMLSchema-instance" />
  </elem1>
</message>

and I need to transform it to this using xslt (prefix changed on nil attribute)

<?xml version="1.0" encoding="utf-8"?>
<message xmlns="http://www.ns1.com" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <elem1 id="att1">
    <elem2 xsi:nil="true"/>
  </elem1>
</message>

I am aware that there is no semantic difference in these two. I have no control over the xml received and am informed that the output cannot have namespace declarations, other than on the root.
I have tried this

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns="http://www.ns1.com"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:i="http://www.w3.org/2001/XMLSchema-instance"
  exclude-result-prefixes="i">
  <xsl:output method="xml" indent="yes"/>

  <xsl:template match="@* | node()">
    <xsl:copy>
      <xsl:apply-templates select="@* | node()"/>
    </xsl:copy>
  </xsl:template>

  <xsl:template match="@i:nil">
    <xsl:attribute name="xsi:nil">
      <xsl:value-of select="." />
    </xsl:attribute>
  </xsl:template>
</xsl:stylesheet>

but I get this from it

<?xml version="1.0" encoding="utf-8"?>
<message xmlns="http://www.ns1.com" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <elem1 id="att1">
    <elem2 xsi:nil="true" xmlns:i="http://www.w3.org/2001/XMLSchema-instance" />
  </elem1>
</message>

The i prefix declaration on elem2 is unnecessary and not allowed by my client.
I can only modify the stylesheet.
Is this possible?

You're copying the parent element and that means (in XSLT 1.0) you're copying its namespaces too.

Try perhaps:

<xsl:stylesheet version="1.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:i="http://www.w3.org/2001/XMLSchema-instance"
exclude-result-prefixes="i">
<xsl:output method="xml" indent="yes"/>

<xsl:template match="@* | node()">
    <xsl:copy>
        <xsl:apply-templates select="@* | node()"/>
    </xsl:copy>
</xsl:template>

<xsl:template match="*[@i:nil]">
	<xsl:element name="{name()}" namespace="{namespace-uri()}">
		<xsl:attribute name="xsi:nil">true</xsl:attribute>
		<xsl:apply-templates select="@*[not(name() ='i:nil')] | node()"/>
	</xsl:element>
</xsl:template>

</xsl:stylesheet>

Please try the following solution.

Input XML

<?xml version="1.0"?>
<message xmlns="http://www.ns1.com" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <elem1 id="att1">
    <elem2 i:nil="true" xmlns:i="http://www.w3.org/2001/XMLSchema-instance" />
  </elem1>
</message>

XSLT

<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns="http://www.ns1.com" xmlns:ns1="http://www.ns1.com"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xmlns:i="http://www.w3.org/2001/XMLSchema-instance"
                exclude-result-prefixes="i ns1">
	<xsl:output method="xml" indent="yes"/>

	<xsl:template match="@* | node()">
		<xsl:copy>
			<xsl:apply-templates select="@* | node()"/>
		</xsl:copy>
	</xsl:template>

	<xsl:template match="ns1:elem2">
		<elem2 xsi:nil="{@i:nil}"/>
	</xsl:template>
</xsl:stylesheet>

Output

<?xml version="1.0" encoding="utf-8"?>
<message xmlns="http://www.ns1.com" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <elem1 id="att1">
    <elem2 xsi:nil="true" />
  </elem1>
</message>