Thank you for getting back to us here in the Community. I'm here to lend a hand with your concern about turning off VAT in QuickBooks Online.
In QuickBooks Online (QBO), once the VAT feature is turned on, there will be no option to turn this off.
As a workaround since you're not VAT registered, we can help him to set the default tax rate selection to choose Out of Scope of Tax or No VAT for all their future transactions.
Here’s how:
Choose the Gear icon.
Tap AccountsandSettings.
Tick Advanced.
Go to Accounting.
Hit the drop-down in Default tax rate selection and choose OutofScopeofTax (or No VAT.)
Hit Save. Then, Done.
The OutofScopeofTax is used for non-VAT registered businesses. This means, if you use it then it will not show in any taxes reports in your account.