Like @Marttt said.
Server create (some) events and send those to player like a announcement, then events appear in client game and perhaps trigger something in client game.
You can block these announcement to clients, but it not work on every event.
Events appear when something is already happened.
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In your example,
you can't stop server side POST event by blocking PRE hooked event.
But returning
Plugin_Stop will block
PRE hooked events, even from other plugins (with same event "name").
It depends in which order plugin is loaded, then it will block calls after that.
*edit So, don't use it
