Adding close buttons to the article tabs is on our list, but probably not something we'll have short term. In the meantime, you can use one of these methods to delete the article:
1. The delete button on the toolbar
2. Right click article and select 'Delete' from the popup menu
3. Use a keyboard shortcut to delete the current article. The default shortcut is Ctrl+Alt+D, and you can configure the shortcut using Advanced Toolbar Customization, under the Tools menu.