Hi there @Retroruud!
Don here from Shopify. :)
Thanks for outlining your position here on our community forums.
If you have Shopify Payments active on your store, even where you have set it to only allow payments via iDeal, there will be an option on your checkout for customers to pay via card.
If you have not allowed card payments then customers in this situation would not be able to complete their order via card as it will just fail.
This is not something that it is possible to edit here, due to the secure nature of the checkout and payment gateways.
As an alternative, if you only wish to capture payments via iDeal you can instead add the iDEAL via Mollie gateway from our list of payment gateways available in the Netherlands.
Think about which of the above options you would prefer to maintain for your store and you can move forward with that. :)
How are things going for your business otherwise? Have you sourced some products and thought about who your target audience will be and how you might reach them?