I am afraid that RadEditor does not provide such a built-in functionality, but you could submit a feature request for such feature and we will consider its implementation. The only possible workaround that I could suggest you is to implement your custom content filter in order to strip the undesired timestamp attributes from the Editor's content. You can see a sample implementation of a custom filter in the following live demo: Custom Content Filters
Note, though, that such filters will be executed when the user is switching between the Editor modes (Design/HTML) or when the content is passed to the server, but no everytime after a new comment has been inserted.
Hope this helps.
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