Not very convenient. In each forum, select "Move/delete posts" from the menu. There, in the filter, select the appropriate settings. In 4.4, there were sometimes glitches, so it's better to first move the topics to a temporary section, check, and then delete them from there.
P.S.: I CRON use a simple php script to search for topics with 0 messages once a month and change them to a hidden section "for deletion". 👨🍳