Yep. The little (!) in the screen with the post, above the quote button, is a report button. Click it and choose a reason for the report from the drop-down. You can also wright a note if you want to, but you don't have to.
This was totally not obvious to me at all. I never would have figured that out. I guess I sort of saw the "!" in my peripheral vision but was also kind of ignoring it. Thanks pb!
(We can report messages for being off topic?! That could be like half the messages people (including myself) post!)