By the time the event is generated, the page has already been created and saved to the database. If you want to prevent a page from being saved based on certain conditions, you need to execute some custom code before the page is saved, before the doeditpage.action.
I think the 'easiest' way to do this (kind of a hack!) is to write a servlet filter plugin module that filters any incoming requests to the doeditpage.action and, if a condition is detected that should prevent the page from being saved, short-circuits the request.
A more 'architected' solution would be to use the Conveyor plugin to override the doeditpage.action with your own implementation.
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