Thank you for the nice words :)
As for blocking the screen - this is not the intended purpose of this control and I'm afraid it isn't possible. The closest you could get would be to have a notification that would require the user to close it manually (AutoCloseDelay="0" KeepOnMouseOver="true" ShowCloseButton="true" ) and attach a handler to the OnClientClose event or the button click.
Please note that this is outside of the scope of this control, yet If more requests for a modal functionality arrive for the RadNotification we will create a public PITS item and if many votes are accumulated we will consider implementing this functionality.
All the best,
the Telerik team
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