>>32
If you have a shitload of open ports, it might give you a false positive. Solution? Don't use condoms with holes in them.
But the best, simplest method I've come up with is simply leaving everything disabled until someone comes along and tries to spam the board. Then ban the IP. He knows how to use a proxy? Get your continually-updated known proxy and tor node lists and run them through the banpanel. (After doing that, use a bit of SQL to delete all posts by currently banned IPs that were made within the last 'n' minutes. Much simpler than deleting the posts by hand.) Still having problems? Turn the proxy checker on. STILL getting flooded? Well then, you have a first-class hate machine...
then enable the captcha, increase the time-between-posts to something like 5 minutes, and keep your finger on the ban button.
This way, the average user doesn't have to deal with captchas at all, unless they're really and truly necessary, anyone using a proxy legitimately is only minimally inconvenienced, and as an added bonus, you'll get a nice fat collection of proxy IPs for future reference. If they come back later and discover that you've gone back to normal operating mode, all you'll need to do is re-add those IPs to the ban list. Chances are, they'll try the same proxies again.