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I think if they don't get paid a couple of times most of them will just spam harder or in different boards.
Maybe, but that's why I said there would be some short term pain. Besides, any other solution than us reporting entire threads for deletion will require changes made to the staff and/or forum (e.g. - hiring more mods).
I regularly report spam mega threads and they always get locked or trashed. I don't think the issue here is the mods not trusting us. I think simply having a dedicated board mod who would monitor threads and regularly lock or trash then before they become spam mega threads would be a big step forward.
The thread I provided as an example above didn't start off as a spam megathread, but it certainly became one at some point in its 58+ pages, hence I don't think preemptively nuking threads is the right approach. Also, there won't be any pain suffered by the sig campaign spammers unless you let them spam for a bit first.
What clearly isn't going to work is what @Lafu did - reporting hundreds of posts in one thread. That's way too much work for all involved and, besides, this is a fail in principle: we should not be spending as much or more time fighting spam as it would cost to simply read it.