For the old DockingManager control, you can use DockingStateChanging event and cancel operation for TabbedDocument state:
sender, DockingChangingEventArgs e)
(e.DockObject == myToolWindow && e.DockableState == Telerik.WinControls.Docking.DockState.TabbedDocument)
Please note that in general our DockingManager control is no longer supported and its implementation will be removed from our suite in Q3 2011. I would strongly recommend updating to your latest releases where many issues have been addressed and many controls have been greatly improved.
I hope this helps.
All the best,
the Telerik team
Q2’11 SP1 of RadControls for WinForms is available for download (see what's new); also available is the Q3'11 Roadmap for Telerik Windows Forms controls.