<% if Menu(2) %>
<% if ID != 11 %>
<ul class="subNavigation">
<% control Menu(2) %>
<li class="liSubNavigation $LinkingMode $FirstLast">
<a href="$Link" title="Go to the $Title.XML page" class="aSubNavigation $LinkingMode $FirstLast">$MenuTitle.XML</a> <% if Last %><% else %> | <% end if %>
</li>
<% end_control %>
</ul>
<% end_if %>
<% end_if %>
There are a few parts to explain.
We check if the current page id is not equal to 11:
<% if ID != 11 %>
...
<% end_if %>
We add the property $FirstLast to the class of the <li>/<a> which returns first or last if either of these cases are true. Note, this will make the class first, not firstItem as requested.
http://doc.silverstripe.org/sapphire/en/reference/built-in-page-controls#and-lt-if-first-and-gt-and-lt-if-last-and-gt-and-lt-if-middle-and-gt-firstlast
We add the dash by doing an if else on the last item like so:
<% if Last %><% else %> | <% end if %>
Hope this gives you what you want or at least the tools to start playing around to do what you want.