I remember, on an old forum that I frequented years ago, that we already had this kind of waves of spammers. We had realized that their IP address systematically came from India or Pakistan, and the administrator had set up a system where for all new members who registered with an IP from these countries, the administrator had to manually validate their account before they could post. Of course, in the registration form, there were a few questions that were asked, a bit like on Facebook groups, to try to sort between spammers and real people.
I don't know if these spammers come from a single country or from many different countries, but it can be a solution. (if it's technically feasible)