External page rendering is made by design and is not considered as an issue which should be addressed. However, we will log this as a feature request - to make external page rendering optional.
For the clients who do not need this functionality, there is a
possible workaround but its logic would be
a bit complicated. You can check the PageViewer template stored in ~/YourProject/Sitefinity/Admin/ControlTemplates/Pages/
which is used for page preview. We use iframe for visualizing it.
If you can modify the code to verify whether
the page is external for your domain, you should be able to prevent
external pages from displaying. This is how only pages that are on localhost will be previewed. If
you have the needed resource you can try modifying the PageViewer.ascx to fit your requirements.
However, please have in mind that we have not tested this scenario.
Hope the provided information is helpful.
the Telerik team