As you can see on the image below, I have got a large space in the panel. In the preferences panel of the panel there is nothing here between the notification area and the battery control applet.
How can I delete this ugly space ?
You haven't mentioned which version of Lubuntu you are using but let me say that I encountered this problem in Lubuntu 12.04 but not in Lubuntu 12.10.
It was discussed quite a bit over here: Empty spaces in lxpanel when using xfce4-power-manager.
A relevant bug appear to be: empty space between time applet and nm-applet.