If a customer chooses an unavailable variant combination, the 'Add to cart' button will fade and update to read 'Unavailable':
If you select 'Color swatches' as the Option style (Theme settings > Product grid), the variant will become crossed out:
Hiding the unavailable variants altogether is not a default feature to Shopify themes. In order to make this happen, custom code would need to get added into your theme (with the possibility of additional modifications).
In one of Shopify's forums, they have a customization post called link product options, which may be able to help.
However, we recommend reaching out to a Shopify Expert if you're looking to make this sort of modification as we cannot assist from a support perspective.
Alternatively, if it's possible that you may be able to get this variant combination of your product in stock, you may want to keep it as an added variant with the inventory level set to '0' so it will then show as a 'sold out' product:
This way, you can enable the built-in Back in stock form which will help you gauge demand and interest from your customers if you're on the fence whether or not you want to bring in this variant combination!
The 'Back in stock' form can be enabled in Theme settings > Product grid > Product form > Enable Back in stock request form:
Which will then look something like this in your shop:
