SPAM can be a lot of things: useless or pointless posts, especially anything whose sole purpose in life is to hike up your post count to get a broom, like lightning rounds. On the forum boards, campaigning for prefect, quidditch captain, etc. will be considered SPAM. SPAM will lose house points!

One word post, multiple one word posts, simple "I agree" posts, creating new threads that ask questions that have already been answered in the FAQ, posting in topics that are over a month old, discussing your broom, your lack thereof, the arrival of your new broom, the impending arrival of a new broom, etc., etc… is all considered SPAM, but is by no means an exclusive list. If you're not contributing anything to the discussion, don't post! You may lose points for it. At the very least, you risk irking your fellow schoolmates and professors. Remember that the boards will soon be managing 3000 registered students.