you can add a element to your element, in this element you unset the token on which your drilled down panel is depending, so whenever you change the input, the token gets unset and the subpanel disappears
<form>
<fieldset>
...
<input type="dropdown" token="xx" searchWhenChanged="true">
<choice value="*">Any</choice>
<default>*</default>
<populatingSearch fieldForLabel="xx" fieldForValue="xx">.....</populatingSearch>
<change>
<unset token="yy"></unset>
</change>
...
<row>
<panel>
<!-- main panel wit drilldown-->
...
<drilldown>
<set token="yy">$click.value$</set>
</drilldown>
...
</panel>
</row>
<row>
<panel>
<!-- drilled down panel -->
<table depends="$yy$"">
...
see http://docs.splunk.com/Documentation/Splunk/6.2.4/Viz/tokens
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