QB sent reminder notices without my automatically requesting or manually requesting it do so: how can I turn that off?

Hello there, handymanhelper.

It could be that the Automatic invoice reminder feature in the settings is turned on. We can go ahead and double-check by following the steps below:

1. Go to the Gear icon.
2. Select Account and Settings under Your Company.
3. Within the Sales tab, go to the Reminders section.
4. Turn off the Automatic invoice numbers feature.
5. Click Save. Then, Done.

Here's a sample screenshot for additional reference:

To learn more about personalizing your sales form, you can check this article: Customize invoices, estimates, and sales receipts in QuickBooks Online. 

Let me know if you have other questions. Take care always and stay healthy.