utilizing the spam filter like that is only going to block that line of text meaning it will only block an email if it contains that exact subject everything else will pass through. I would suggest blocking the domains or email addresses.
You'd be suprised at blocking spammers for a long time then unblock them and find that all spamming has stopped. I think what they do just like any other spammer out there is that they send a ton of emails from 1 account close it then create another one and move on. Just keep blocking the domain or address if they are using a domain like verizon.net yahoo.com hotmail.com etc.