Only Approved Ideas could be converted. Play with Idea Status and see if you give some different status (Probably you could introduce new one for your purpose) and see if that takes away 'Convert' button. In that case, you could create a process or trigger to update status once Idea is converted.
There is XSL option but not sure you could configure it to hide it for some conditions:
Location: /opt/niku/clarity/META-INF/pma/vxsl/idea
File: ideaProperties.xsl
<xsl:if test="/data/ideaRights/@showConvert='true' ">
<action type="button" id="convert" actionId="pma.ideaConvert">
<label value="Convert" xml:lang="en"/>
<data id="id" value="{id/@value}"/>
<xsl:if test="string-length($returnTo) > 0">
<data id="return_to" value="{$returnTo}"/>
</xsl:if>
</action>
</xsl:if>