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.
Almost any thread in Bitcoin or Altcoin Discusion could develop in to a spam mega thread, regardless of the topic. The spammers are just there to churn out a line or two. They don't read and don't care about anything that came beforehand.
In the example you gave, even by page 3-4 there is no discussion going on, and the spammers have turned up with their usual great insights:
I think to block bitcoin sites need to be revisited in terms of governance because not a few people are dependent on bitcoin
no government, however rich and authoritarian, can totally ban the use of coin. Bitcoin is unstoppable.
There are so many people missing out here in the forumand all investors will lose their funds in bitcoin of course and will also lose bitcoin in banks I think it's great to suffer and lose work like a bounty hunter.
Bitcoin is such a big help for us it gives an extra income so i can pay some of our monthly bills. I can't imagine if our government will ban bitcoin.
This is the kind of thread that could have been nipped in the bud by a dedicated board mod looking out for these kind of things. This thread could have been locked on page 5 without losing any quality content. Instead, it remained open for another 1000+ shitposts.