How do I remove the banking details appearing in the email message that a client receive?

Great to see you in the Community, @Arazeh. I can help you remove your bank details on your sales forms in QuickBooks Online (QBO).

You can find the payment information on your sales form. You can uncheck the bank details and online payment in the box to remove it.

However, you can't remove it if the form has been sent without unchecking the online payment details. I'll show you how to do that. This time we are going to use an invoice.

1. Create an invoice.
2. On the invoice page, uncheck the Online payments options.
3. Fill in the fields and select Save and send.

Moreover, I recommend checking this article to learn how to design and personalize your sales forms in QuickBooks Online in the future: Customize invoices, estimates, and sales receipts. 
 

Feel free to tag me in the comments if you have other concerns about customizing sales forms in QuickBooks. I'd be happy to help you some more.