They can get activities by posting in latest topics as well. Why would they do it in those very much old threads?
And, signature campaigns should be limited to Member+ accounts. It has been said a lot of times before but isn't enforced yet. It would definitely reduce spam to a very large extent.
Saw one again today:
The last reply on that thread was about 8 months ago, in December 2017, and people, I don't know from where, have found it and started posting in it again just like it has been started yesterday. I don't really understand the mindset of such people who don't even bother to read the starting post before starting to reply, or maybe they read the starting post but just don't see the date it was posted on.
This is the thread I'm talking about here:
https://bitcointalksearch.org/topic/hi-there-people-im-new-to-the-community-2610924