Thanks, I am sure this should be drop-dead easy, but our UBB Classic 6.5 is on Unix and our one Unix guy is no longer here.
Anyway, necessity is the mother of invention, so I may have solved the problem myself:
All of our spams sent to us via "Report Post" referenced the same [valid] topic and posting. I assume that the spammer saved the "Report Post" URL and it was still functional even after I disabled the Report Post option on the board.
So, I closed the board and moved the entire topic to another board, deleting the original topic, and then moved it back again, yielding a new topic ID. The old topic URL is now invalid.
Since I did this this morning, I have had no more spams (as opposed to receiving easily 50 per day!) So it seems to have worked at least as of now.
I still plan to keep the Report Post option disabled, but, again using only UBB Classic 6.5 features, I rejiggered the UBBFriend feature to do double duty as a way for members to report truly inappropriate posts as well as just sending a topic to a friend.
I revised the ubb_friend_intro Email WordLet entry to include instructions on how to use the feature to email us the page if it has an offensive post on it (including correct email address to insert on the page), plus edited the UBBFriend Primary WordLet entry to specify to click here to email this page to report an inappropriate post, or simply email this page to a friend.
Kludgey, sure, but I believe it solves the problem using existing UBB Classic 6.5 functionality