I've never seen this before, but yesterday I had 4 in-game calendar invites from gold selling websites on one of my characters. I know BadBoy already scans Battle.net invites -- are calendar invites something you'd consider adding?
Last time I looked into Calendar reporting, the functionality was completely FUBAR. Luckily Blizz changed something which seemed to fix the spam invites. I guess they've found a way around whatever Blizz changed, I'll take a look at the API again and see if it's improved at all.
It seems like it's still buggy. The first time I clicked "Report Player" in the right-click menu on the invite, I got an "internal calendar error" message in the UIErrorsFrame. The second time it seemed to work, or at least gave me a "Thanks for your report" message. It occurred to me afterward that it may have failed the first time because the "event" automatically opened its detail panel when I clicked on the player, but the second time it was closed... maybe you can't report the invite while the detail panel is open? I'll try to test this next time I see a spam invite.
If those are in Trade chat, don't bother posting them... Funkydude mentioned recently that he no longer cares about blocking guild spam outside of whispers. I've just added words like "guild", "vent", "mumble", "free", and "perk" to BadBoy_CCleaner, and that seems to block most of it.