For the record, I've had a plugin that does this for awhile. Unfortunately, life events have kept me from getting it updated, but it still does the job nicely.

https://wordpress.org/plugins/seriously-simple-spam-blocker/#description (I adopted this one from Hugh Lashbrooke.)

The thing I like about this new plugin is that it does the reporting back to Stop Forum Spam. That's a feature I've wanted to get written but haven't had the time. Props to Leland!