Actually, you are right. If you put them in the blacklist, then they go to spam right away.
The issue is that you don't want to do that with an IP address as I said since it might inadvertently catch a bunch of innocents in the blast.
The problem I've noticed with spammers lately is even though the IP is the same, they use different email addresses, usernames and websites when they comment so your blacklist section can get huge really quickly. Right now I have 4 spammers that just append a number to the end of their username, and right now they are using 1-9 so I would have to put all of those usernames into the blacklist.
No matter what, it is an irritating thing to deal with. Right now I'm getting anywhere from 50 to 450 spams per day, but luckily 99% of them are caught by the spam filter.