Which version of AspDotNetStorefront are you using? This can be accomplished pretty easily with changes to only a couple files. First, the template.ascx file. You'll need to add two attributes to the ComponentArt:Menu in the template: RenderDrillDownDepth and ExpandOnClick
Code:
ExpandDuration="0"
ExpandSlide="None"
ExpandTransition="None"
RenderDrillDownDepth="2"
ExpandOnClick="true"
CascadeCollapse="false"
CollapseDelay="0"
CollapseSlide="None"
The second thing you'll need to do is remove the arrow from after the top categories. Open the EntityHelper/EntityMenuList.xslt file, and find the if statement around the Look-RightIconUrl attribute that sets the arrow.gif file as the image to use. Then just add a boolean check that only renders the image on the admin site
Code:
<xsl:if test="boolean(child::Entity) and boolean($adminsite)">
<xsl:attribute name="Look-RightIconUrl">arrow.gif</xsl:attribute>
<xsl:attribute name="Look-RightIconWidth">15</xsl:attribute>
</xsl:if>
That's all you should need to do. The dropdown menu will only render the top level entities now, and the menu will only open when clicked (actually the way the component art menu works is that the menu will expand once clicked...and then all menus will expand on roll over until the menu loses focus...eg. the user clicks somewhere else on the page). If you're still getting the error, can you paste the exact error message here?