You typically want this for source namespaces:

<xsl:transform xmlns:xsl='http://www.w3.org/1999/XSL/Transform'
   exclude-result-prefixes='m'
   xmlns:m='http://www.w3.org/1999/MathML"
>

<xsl:template match="m:math">
<p>Math is not supported</p>
</xsl:template>

</xsl:transform>