Autoban is permanent, unless manually removed/modified. Nuke bans and removes all of a user's posts (spambots usually). Manually applied bans won't show up in the modlog, those are usually temporary, but not always.
Given the fact that it's possible to "nuke" a spambot account, ie delete all posts, why isn't this possible for normal users? I've seen other users asking about deleting an account and the only reply is "we don't do that, change your passwd to something random and go drinking".