Re: User / messaging exploit? Causing spam
You know you’ve made it when… LOL 
I would highly recommend against closing off the private messaging system or even allowing it as an option. Being able to message someone you are not friends with is a HUGE use case in my opinion. Crucial even. I wouldn’t give users the option to set it to friends only. Or at least… I would like the site admin to have the ability to disable that option.
Personally… I despise CAPTHCA. Don’t pass your problems off on your users. Like websites that say “Best viewed in” or “Set your screen size to”… etc. Any solution must be invisible to users. I’ve heard of people using javascript events (mouse click for instance) as an alternative. Sounds good to me. Here’s something I found with a quick Google search.
http://www.webdesignfromscratch.com/javascript/human-form-validation-check-trick.php
Alternately… you could use a simple math question… like as in example. LOL
http://farm3.static.flickr.com/2174/2268237733%5Fcda4a1dbb3.jpg?v=0
