The only way to avoid that is to use DestroyOnClose and destroy the window object. The problem here is not with the RadWindow control itself - it is basically an IFRAME and this is the way how browsers work with IFRAMEs. If you load a page in an IFRAME, navigate away from that (content) page, this will result in adding this action into browser's history. For security reasons however, the history object
is read only and you cannot clear it.
As noted in the article above, you could use location.replace() to change the url of the content page without adding an item to the history, but this depends on the developer to change its logic - we cannot control that behavior in any way.
the Telerik team