The target to

build.xml

...
	<target name="undeploy.content" depends="reset.content">
		<property name="temp.dir" value="temp"/>
		<xslt in="${temp.dir}${file.separator}package.xml" out="${temp.dir}${file.separator}destructiveChanges.xml" style="xslt${file.separator}destructive.changes.xslt" force="true"/>
		<xslt in="${temp.dir}${file.separator}package.xml" out="${temp.dir}${file.separator}empty.package.xml" style="xslt${file.separator}empty.package.xslt" force="true"/>

		<zip destfile="${temp.dir}${file.separator}undeploy.zip">
			<fileset file="${temp.dir}${file.separator}destructiveChanges.xml"/>
			<mappedresources>
				<fileset file="${temp.dir}${file.separator}empty.package.xml"/>
				<globmapper from="*" to="package.xml"/>
			</mappedresources>
		</zip>
		<sf:deploy username="${sf.username}" password="${sf.password}" serverurl="https://login.salesforce.com" zipFile="${temp.dir}${file.separator}undeploy.zip"/>
	</target>
...