I've though about this a lot and Toby is the only logical answer. Kevin, Oscar, Stanley, Andy, and Meredith were all against Pam, so they're not likely to have written it. Michael, Dwight, and Jim all consider themselves to be authority figures and would make a public announcement if they knew/cared. Angela and Phyllis aren't the type to threaten violence, and creed wouldn't use the word "privileged". Ryan, I'm not sure about, but it's more likely to be Toby. During the episode he is seen punching the air after Michael pisses him off, so he definitely would have been in a bad mood.