Checkout issue currency

Hi,

In the last 2 weeks we have had 2 times a customer who managed to pay in the currency they chose when visiting the website, but we have set them to pay only in EURO because of our administration.

We do not understand how it is possible that these customers have been able to pay in DKK and GBP. One paid by paypal and the other by credit card.

Is there an additional setting to prevent this?
I have screenshots if needed. I can’t add them directly.

Thank you.