Those only so much we can do to stop these scumbags unfortunately-I've just deleted a whole pile of the spam threads. They target the biggest websites as well and they struggle to stop them-I've seen it first hand.
I've never seen them on sites like imgur or reddit? Maybe you just need more more semi-mods, perhaps with a limited ability to quarantine users (an admin or mod could then decide to either remove the quarantine or enforce a ban) and make threads invisible (an admin or mod could then decide to either make the thread visible again or delete it)? I'd be glad to help out.