Customers checkout in wrong currency

From time to time some customers get into the checkout with a wrong currency. I’ve only activated my base currency for checkout and all other currencies should switch back to the base currency in the checkout, but they don’t. Sometimes customers will go into the checkout with USD, so the amount is obviously higher than with euro, but PayPal then simply converts it back to euro so the customer over pays. Or once a customer got into the checkout with USD and then even got through with it, but the order was placed on hold, since I don’t accept other currencies. So I had to cancel on the customer. I need a functioning converter for display purposes only. I need a fix ASAP. I’ve now deactivated the plugin.