Unfortunately there isn’t currently any automatic cleanup of soft-deleted posts, and no manual bulk option in the UI either. The rails console is still the way to approach this if you need to do it.
There are a few topics with some info in about how others have approached it. I think this one may be a good example to get both topics and posts (though will need a tweak to tailor it for your own use case) How to delete thousands of Personal Messages? - #7 by RGJ. And remember to take a backup before trying anything destructive in the rails console as accidents can happen. 
I think this is the relevant Feature request with the most recent activity if you’d like to add your vote and voice to it: