I'm not misleading anyone but most mega thread contains a lot of spam, compared to smaller ones.
If you lock those megathreads, where do you think those spammers will write? Do you think that they would stop posting or suddenly increase their post quality because of that measures? Of course not, they would just share their nonsense in other threads so I don't really see how what's proposed in your first post would help the forum.
Additionally, some managers use Ninjasty.space to count posts without properly evaluating the posts' quality as long as the user met the weekly post requirement.
Its true, managers could do more to control their signature campaign members, but the problem is that there are far more spots in signature campaigns that there are quality members so if they kick someone (unless its one of the best paid campaigns), they won't be able to replace with anyone better so they just let it slide as they have to fill their campaign somehow.