This is a Trello specific issue, which people can live without knowing when they can just archive cards and forget about them. But, if you are managing your website from Trello it can be crucial to know how to delete cards, since archiving will not unpublish them.
Here's how you do it:
So, in words:
- first open your card in your Trello interface;
- then click in the "Share and more..." link on the bottom of the left sidebar of the card;
- then click in the "Delete" link.