I can understand the confusion now. SetupDocument event should be used for post-processing the document after import and before setting it to the associated RadRichTextBox.
If you want to modify the source itself, it may be more appropriate to do this before the binding occur, for example in the getter of the source property in the ViewModel.
You can also choose to clean the symbols from the already imported document in the SetupDocument event handler, for example using the Find/Replace mechanism
the Telerik team