The only threads that are deleted are ANONYMOUS posts from banned members. You can guess why I do this.
1. They are banned. Probably for a reason.
2. The worst of a banned members posts will be the ones they posted anonymously.
3. Some banned users come back again, and again, and again... I want to discourage this by deleting all their "work".
So I have a script which goes through and deletes all the anon. posts. As well as any up/down votes they ever gave.
Moral of the story: I don't want banned members contributing in a subtle way to the "anonymous zeitgeist" of CathInfo (what is good/bad, what is popular, ways of thinking, etc.)
I don't delete ALL a banned member's posts, because the ones they signed their name to can be judged like any other post.