Off the top of my head, I can think of 3 specific people on PC that really annoy me in this way. One of them contributes virtually nothing to whatever topic's they post in, and they are everywhere, they are always, they are no one.
The second is even worse, and used to post in pretty much every topic in the sub-forums I inhabit, but I haven't seen them for ages, which is good because it got to the point where simply seeing their avatar filled me with frustration. Every post was as minimal as possible, and pretty much blatantly ignored every other post in the topic. I dunno, their posts almost had some kind of ethereal troll quality to them, if I'm making any kind of sense.
The third doesn't appear as often as the others, but the content of their posts, for various reasons, never ceases to irritate me. Everything they post is filled with some kind of misjudged arrogance... urgh.
However, seeing someone's posts EVERYWHERE isn't an inherently bad thing to me. There are quite a few users here whose posts I distinctly look forward to reading. Hope that doesn't sound creepy!