With copy-of the choices are all or nothing, I'm afraid. If you want to be more selective, you can do the copy using a recursive shallow copy, in which case you have more precise control over the namespaces:
<xsl:template match="*" mode="copy">
<xsl:copy copy-namespaces="no">
<xsl:copy-of select="namespace::xsi"/>
<xsl:copy-of select="@*"/>
<xsl:apply-templates mode="copy"/>
</xsl:copy>
</xsl:template>
Michael Kay
http://www.saxonica.com/