Ken - I'm obviously not Nabble support. But, I changed the permissions (Options/Users/Change Permissions) on my forum such that anyone can register, but only members can post - anywhere. You can read about how I did it starting here, but it has been very effective. Since implementing it on November 1st I've not had a single instance of spam.
Thanks, Gary. For now, I have simply removed posting permissions from anybody ranking "lower" than members.
Since my forum has been wide-open till now, it actually has no members. Is there a way I as admin can make my "real" (i.e. non-spammer) registered users into members? Or is that something each user has to do for him/herself?
But under Options/Users/Manage Users & Groups you add each of the Registered Users you want to be able to post into the Members category. Just copy and paste them in. But don't forget to hit Save when you are done.