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The Charge Command
What should be simple, is far from it.
Tesla vehicles have both a charge_enable_request and user_charge_enable_request value in their charge_state data.
When controlling charging directly, you are influencing the user_charge_enable_request value, however this value will automatically unset when you unplug the vehicle.
This means you can prevent the vehicle charging before plugging in, but you must do so every time. A way to avoid having the vehicle charge when plugged in is to set a charge schedule, or lower SOC charge limit.