Q:

How to delete all conversations all at once in iMessage?

It's clear on how to delete conversations in iMessage one by one. Does anyone know how I can delete multiple or all conversations all at once? Thanks.

A:

In Mojave the keyboard sequence is to first delete the message with Command + Delete And then keep Command pressed and press d 
You can't delete multiple message at once unfortunately, but keeping the button Command pressed and alternately press Delete and d makes the delete process a lot less cumbersome.

A:

There is no way to delete multiple or all conversations from the Messages app at once. However, there's a keyboard shortcut that you can employ to speed up deleting multiple conversations in a short duration of time.
Upon selecting a conversation, and hitting Command + Delete brings up a confirmation dialogue as shown:

Now we would be back to square on if we attempt to click through or select Delete button and confirm for each and every conversation. 
However, the Delete confirmation intent can automatically implied by using the keyboard shortcut Option + Command + Delete. Holding down this key combination, one can quickly zip through deleting multiple conversations quickly.

