Lock the thread; create a self-moderated thread
It's impossible for everyone to start creating self-moderated threads and you can't force people to do it, if they simply don't want to do the moderating job.
You can have many threads like this but this won't change a thing if creator is not active and deleting stuff all the time.
This can have more negative sides with many members avoiding to write in self-moderated topics.
A new thread should be created at the beginning of every season. Old thread locked.
This would only create more spam, and it's just bandage solution that fixes nothing.
Creators of many topics are not active members so that would create a mess of many duplicate topics in forum.
Admin to convert the already existing thread to self-moderated.
I will repeat again that you can't force someone to self moderate thread if he doesn't want to do it.
Will a neutral tag with a bold message to these users be sufficient to send a signal to sig managers?
Good managers are not blind and they can see who the spammers are.
You can report all post you consider to be spam, that is how you can improve forum.