Is there an option to turn off automatic update of currency exchange rates?

Welcome to the Community, Adrian. I'm here to provide you with some information about currency in QuickBooks Online (QBO). 

In QuickBooks, turning off the automatic update of the currency exchange rate is unavailable. However, you can still manually add your currency within the Gear icon.  

Here's how: 

1. Go to the Gear icon.
2. Under Your Company column, click Account and Settings. 
3. Scroll down and hit the Advanced menu. 
4. Find the Currency section and click Manage Currencies. 
5. At the top right, click the Add Currency option. 
6. Within the Add currency dropdown, choose a country. 
7. After selecting your country, it will be added to your Currency column. You can now edit it by clicking the Edit currencies exchange under the Action column. 
8. You'll see the Market Rate and Your Rate. Click the Your Rate radio button. This way, you can enter your preferred exchange rate.
9. Press Save. 

You can refer to this material if you want to learn more about the use of your exchange rate for multiple foreign currency transactions: Enter and use your exchange rates in QBO.

Furthermore, you can also learn about exchange rates in QBO.

If you have questions or information regarding exchange rates in QuickBooks, kindly enter it in the comment section. Stay safe.