I agree with mikeash here. If a hellbanned person still wants to remain part of the HN community, it's really not that hard to create a new account. Sure, sometimes they get it wrong, but the vast majority of the case, they get it right.
That means, that by you turning on dead posts, you are subjecting yourself to the 80% case where they get it right. If one of them is spamming with huge blocks of text, etc... well, you signed up for it in the low-probability case that one of the dead posts actually contained valuable information.
Keep it simple. Don't start adding multiple layers of hellbanning protection just to fix a low-probability edge case.
That means, that by you turning on dead posts, you are subjecting yourself to the 80% case where they get it right. If one of them is spamming with huge blocks of text, etc... well, you signed up for it in the low-probability case that one of the dead posts actually contained valuable information.
Keep it simple. Don't start adding multiple layers of hellbanning protection just to fix a low-probability edge case.