I think it's already been said, but people who have every other post on a forum game.
I get that you can be enjoying the game and want to take part, or people are slow to reply so you reply as soon as you can, but it's terribly boring for others to read.
Also, it can be quite annoying if that person has a conflicting sense of humour or is just disruptive to the community effort in playing the game. Sometimes people need to space their posts out a bit. Especially if it's a quick topic, or requires little content in each post.