[@johnhiler Member 15 years, 6 months ago]

There’s no official support for this… but someone wrote a script a while ago to unofficially pull this off for 1.0:

https://bbpress.org/forums/topic/permanently-deleting-topocs-and-posts

I haven’t tested it though and it hasn’t been packaged as a plugin. If you try it out, definitely backup your database first – and test carefully!