remove lines between menu items

Hi,

When I use menu’s with subitems in the sidebar, a line appears underneath each subitem, just like underneath each main item. The line that normally appears at the bottom of each main item remains and because of that, between a last subitem and the following main item a double line appears. (ebla.nl/ebla)

Is there a way to remove the lines underneath the subitems, so that only single lines will be shown?

Thanks in advance,
Marianne