When configuring Jetty XML you can set showContexts to false on the DefaultHandler.
If you are using older Jetty versions replace org.eclipse.jetty on my example with the old org.mortbay.jetty package structure.
<Configure id="Server" class="org.eclipse.jetty.server.Server">
<!-- =========================================================== -->
<!-- Set handler Collection Structure -->
<!-- =========================================================== -->
<Set name="handler">
<!-- the collection of handlers that will handle the request -->
<New id="Handlers" class="org.eclipse.jetty.server.handler.HandlerCollection">
<Set name="handlers">
<Array type="org.eclipse.jetty.server.Handler">
<!-- primarily handles the request and maps the request to a ContextHandler -->
<Item>
<New id="Contexts" class="org.eclipse.jetty.server.handler.ContextHandlerCollection"/>
</Item>
<!-- The default handler ... handles the request if not yet handled -->
<Item>
<New id="DefaultHandler" class="org.eclipse.jetty.server.handler.DefaultHandler"/>
</Item>
<!-- The handler for your request logs -->
<Item>
<New id="RequestLog" class="org.eclipse.jetty.server.handler.RequestLogHandler"/>
</Item>
</Array>
</Set>
</New>
</Set>
<!-- ===================== -->
<!-- DefaultHandler config -->
<!-- ===================== -->
<Ref id="DefaultHandler">
<Set name="showContexts">false</Set>
</Ref>
</Configure>
Maybe you'll also be wanting to prevent directory browsing configuring the DefaultServlet of your web.xml,
<servlet>
<servlet-name>default</servlet-name>
<servlet-class>org.eclipse.jetty.servlet.DefaultServlet</servlet-class>
<init-param>
<param-name>dirAllowed</param-name>
<param-value>false</param-value>
</init-param>
<load-on-startup>0</load-on-startup>
</servlet>