It's taking up more and more of my time getting rid of spam. Something must be done!

Currently (if the technology works as advertised) you won't be able to do much on the forum unless you've already contributed by making at least one post. I'm thinking of adding the requirement to have parted with some money before new users are permitted to post, or download anything useful. This restriction will NOT apply to anyone who has already posted.

What are the pros and cons of this approach?