Updated June 12th, 2019.
You may have tried pressing Delete or Backspace or even the forward delete key to delete a conversation in the Mac’s Messages app. If you’ve tried, you know that none of that works. What does work is to hold the Command key (⌘) down while pressing Delete or Backspace (forward delete won’t work).
If you select a conversation (see below) and then press Command-Delete (⌘=Delete)– or Backspace– depends on what your keyboard calls it–you’ll get a message asking whether you’re sure you want to delete the conversation.
You might think this dialog box requires you to click the “Delete” button by hand. Turns out there’s a way to do it from the keyboard. The secret: Command-D (⌘-D). Command-D “clicks” the Delete button, as if you’d done it with the arrow pointer.
Note: there was a time when the confirmation dialog’s default button was Delete– see below, a screenshot from a long time ago! Notice how the blue button (the default button, the one you can “click” by hitting Return or Enter) used to be “Delete” and also notice how now it’s “Cancel.”
Holding the Command key down and typing the first letter of the button you want to click often works in other situations and apps also. It’s always worth a try.