03-27-2014 04:29 AM
pay attention to the words I've marked in bold in the first answer from the technical support member:
"From the BlackBerry Hub, hold your finger on the date at the top of the list. A side menu will appear. Click on the Garbage Can to delete all messages prior to that date"
At first, I also didn't understand how to make it works, but you have to slide until the date is at the top of displayed messages, then you can tap and hold to select all messages preceding that date.