Well I dropped membership for reasons of my own, but posting as a guest, I'd be perfectly happy if guest posts were not automatically posted, but moderated before they appear. If this means a long delay (depending on willingness of moderators), so be it. That might mean that guests only post if they really need to.

Another alternatives is the scrambled characters box used by many open- posting forums and download pages- you know, the box that says "type the characters displayed" and shows half a dozen randomised letters and numbers, distorted so that they aren't machine readable.