It’s possible that those 5 people remember those posts being there and might want to find them later for reference.
Here’s another question. What if the user thinks he’s making a good post, and can’t figure out why his posts aren’t there any more? Maybe he figures they just disappeared into the æther? Server error or something? Is he likely to get into more trouble if he keeps trying to make his point in the same fashion instead of addressing whatever the problem is?