Actually, mere moderators CAN in fact ban someone instantly and permanently...by custom infractions worth enough to get someone banned...and I think there's a special 20 point infraction for people stupid enough to post hentai/porn/explicit images on the site.
Of course they cannot use that infraction for just any reason, under threat of losing their status...but if a spambot comes along, floods the forums with porn pics...that is largely why they disappear so fast...because if the mod of one section is on, and it hits their section, they've got clearance to drop the ban hammer IF AND ONLY IF there isn't an S-Mod+ to do it for them. I've seen it done before, and I've actually been on the receiving end of such an infraction from an abusive moderator. (who was later relieved of his duties)