At present we do not have an additional property whether to show or not the separators. Nevertheless, your feedback is very constructive and I think that such a property will be helpful not only for separators. We could apply the same logic to items in the toolStripItem that we want to always be visible, and which could not be removed by the overflow menu. What do you think about that idea? I think that we will manage to add this functionality in one of the next versions.
I'm looking forward to your reply.
the Telerik team