A libertarian will argue that fraud is an act that is not the same as free speech and hence is not covered by free speech. I don't agree with the libertarian viewpoints overall, however I will acknowledge they have a point about the distinction between those two things.
And what is spam? I can acknowledge fraud not being free speech (although it's a stretch as well), i.e. it's intended to harm (again one could argue about a lot of other speech as well), but how can they make the same argument for spam?
We are in a discussion about free speech absolutism. Somebody who says they are a free speech absolutist who supports spam filtering, needs to either justify why spam is not speech (which is my question, why would it not be speech), or acknowledge that they agree there are limits to free speech.
Or is your argument that if it is commerce it's not free speech? That would allow all commerce to be censored (and we just kick the can down the road, what is commerce).
Spam can be whatever who so happens to be the site moderator, or maybe some very tiny yet obnoxiously vocal minority, wants it to be.
Beyond that practical matter, the least bad definition I have for spam (and this is just off the top of my head right now) is: advertising that is unsolicited and disseminated in a bad faith style.
Any of us who are being honest with ourselves, maybe with exception to the acutely socially challenged, know spam when we see it.