Currecy by country is not working correctly.

I am trying to force the currency to change depending on the billing address, with some locations forced to GBP and some locations forced to EUR as shown in the screenshots. 

The system is working perfectly for the countries that use EUR as default, but it is not working for countries like Norway, Poland, Sweden and Switzerland that do not use EUR as their default currency. For these countries, the customer is able to override the EUR forced currency and pay in GBP, which is incorrect.

I believe that the plugin has an error and the force currency option is not working properly. How can I ensure that the forced currency cannot be overridden, so that customers with a billing address in all the countries in my EUR list can ONLY pay in EUR?

Best regards and thank you for your help!