Any campaign manager would surely spot a spam post and besides they are only paying like 20 post max.
If a poster is only posting just to get paid from a signature campaign, then thats their problem as sooner or later they could be removed from their campaign. Anyone that is bothered with spam post could always chose to ignore such user.
By the way, isn't it that ranking up has to do with the time you spend in here and not depends on how many you have posted?
Sure you can define one liner posts. Something such as "Great Post", "I agree!", "+1", etc. Those are crap one liners... you don't see them near as much anymore as they lead to a ban. But look at posts people have posted as little as "+1" saying they agree, and it really does nothing for thread. It just is clutter.
Spam is based on content not the size. I can think of fixing problems on hardware one line of anwser might fix some problems. It's helpful, fixed a problem, and less then 4 sentances. So 4 line rule is not 100 percent.
If you really want to look at "spam" go over to the off-topic board. I avoid it as it's full of it. Things like last song you listened to, last thing you ate, last thing you drank. Off-topic is where you will find a ton a crap posts (and yes there are some good). But there is a lot of spam in that board, which is why good signature campaigns don't even pay for Off-Topic.