Certain core events are fundamental to how the service operates - things like login, purchases, etc. Those cannot be disabled.
But bear in mind that it's not millions of events - it's millions of units on the event meter. Each event ticks the meter 1 for every KB of the event data (https://docs.microsoft.com/en-us/gaming/playfab/features/pricing/consumption-best-practices).
By a wide margin, the largest events for most titles are Cloud Script execution events, which are optional. They are only sent if you explicitly set the flag to send them to true, which we recommend you not do in production (it's really only meant for debugging). Having it as an optional parameter you can turn on for a specific client (using a title data stored list of IDs, for example) is a good best practice.
You can also turn off all optional data reporting (system info, focus changes) in the Title Settings->Data Collection tab of your game.
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