Disassociate unrelated message threads in Mail.app?

There are some folks who insist on sending me e mail with either no subject lines or very vague subject lines.

What this ends up doing is associated messages with no subject or vague subject lines together when they really are not related at all. I've attached an example; that four different conversations from four different people, and none of them are related at all, but they are all part of one thread.

Anyone got any ideas about how I can fix this easily?

 

Yeah, I did, but I don't mind the threaded messages. Primarily, I'm concerned with message threads being mixed with each other when they don't belong together.

Thanks, though, for the response!