Hi marco rossi,
Thank you for contacting us.
It is possible to disallow docking of a floated window, with a few lines of code.
Each ToolWindow has a property AllowedDockState with in you case you should set to anything else but "Docked". However RadDock still does not offer functaionlity like for example Visual Studio for Floating Windows and you should manage the value of this property manually.
You may also take a look at the eventa RadDock.DockStateChanging/Changed wich can be used to track dock window states or to cancel a docking operation. However currently there is not any way to prevent docking form showing the docking guides.
We are considering enhancing this functaionality for the upcomming SP1 and making it more the "Visual Studio" way, where you cannot dock a window back if it's state was set to "Floating" from the context menu.
I hope this info was usefull. We will appreciate any feedback you may have about this feature or RadDock as a whole.
the Telerik team