Force customer to choose variant

I have set up a number of products, most of them with variants. Usually I set a default value called "make a selection", but this is sometimes ignored, so I have to ask the customer by email after ordering.
How can I force customers to make a selection? I have checked the forum, but could not find too many hints. I believe one solution was to set the amount of the default variant to zero using the Manage variants option for the product, but that did not work, the amount of zero is ignored and customers can still order without making a selection.
I do not have a link to this, I am testing on a localhost.

Yeah, ok, that's obvious, did that. I sort of managed to change the behaviour by removing the option, re-applying it, and then setting the quantity to zero of the default variant.

However, what happens is that the "Add to cart" button is removed, and the message that the product is out of stock is displayed. Of course I can override the translation to get a more fitting message, but I would not want the button to disappear.

Basically I would want to show my page as exactly as it currently is (like for example: hielspoorinfo.nl/profesionele-hielspoor-zool ). Only when an order is placed I would want a message that a selection should be made.

Is this possible?

Custom fields might be the way to go for most of my products indeed, thanks, didn't know that they could be used that way.

However, I would also like to achieve this for products with product-options with varying price, like this one: hielspoorinfo.nl/bauerfeind-viscospot-hielkussen
Is this possible? Or do I understand it isn't (without custom code)?