Yes, it is VERY irritating.
I might be watching a specific thread, and see a new message, but it's just one of the spam things!
I use some other forums, and usually there is someone who can do something about this, either delete a post, or move it (i.e. keep a thread headed SPAM and move all such non-midi posts into that thread, so that the people who post that sort of thing can happily read each other's spam to their little heart's content).
Maybe the forum software does not allow such things?