Unless Apple is lying:

On iPhone, Settings → iCloud → Storage → Siri → Disable and Delete

Edit: Tried it. It works for me. Takes a minute though.

I have a current case open with Apple with this issue. It does not work. And I don’t believe you. I’m sorry I just don’t believe you because Apple says there is a technical problem preventing this. That does not just affect me. Because I also tried it on three other phones of three other friends of mine and it does not work.

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Noaidi 13 hours ago | parent | prev [-]

TRY IT!

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runjake 12 hours ago | parent [-]

I did. It works, as far as I can tell?

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Noaidi 8 hours ago | parent [-]

You’re able to disable Siri in iCloud? Not turn off Siri, disable Siri back up in iCloud. And when you go back to it after not turning it on, it’s still off?

	▲	runjake 8 hours ago \| parent [-]
		Yes. But, I'm not surprised that you're having issues. In my experience managing a large enterprise, iCloud accounts seem to have all kinds of weird, account-specific issues.