IMO - It has to be one of the most effective solutions if the OP can no longer prevent spammers from bombarding non self-moderated thread. Meanwhile the OP can also report some spam posts in the non self moderation thread if he doesn't want to create a new one.
Some projects and their representatives in forum do the same too.
They created a new announcement topic, announce it in a past topic, lock it and move onwards with a new one, that is self-moderated.
I also see some projects do an opposite: initially they create self-moderated topic, later after community complains, they locked it, and create a non-self-moderated topic. I believe they did not know about the procedure I shared above (move it to Meta the move it back to where it belongs to)

They may not know how it works. Some of them don't really care about this kind of stuff
so it certainly makes sense why they'd make a new one and lock up the old one. Another reason, they don't bother to care about how the forum works is because they tend to think about developing the project.