I'd have to agree with what Caelstis. Sure, blocking doesn't completely get rid of the person or punish them, but at least they leave you alone xD
Sometimes if you don't block the person, they're either disrupting an rp or chat you are having with friends, which could get very annoying. You can ask them to leave but people for some reason get VERY offensive about it, even if your asking nicely (had this happen to me a few times) :/
This can also prevents arguments you may start between the person, and no one's time has to be wasted ^.^ (very useful for trolls).
I had to choose annoyed, because that fit the description for my attitude toward all of the reasons that were listed haha.