I've banned a lot of spambots. But the point being, they were bots, so the best method of dealing with them is to keep them from registering, to begin with.
Is the captcha an effective filter in screening the bot's registration? Another question, is the access of bots in the forum does not constitute a hit? What I'm driving at is the hits count of the forum that increases when the bots come around. So in terms of hits count, the bots are contributing. Am I right on this assumption?
Not quite. It's actually the other way around. CAPTCHA is broken right now. For example, Google's reCaptcha was good for a while, until xrumer or similar programs found a way around CAPTCHA by putting all answers into the actual program itself. Likewise, websites such as "StopForumSpam" has been gathering a database of IP's, E-mails, and other info to help forum owners fight against spam in conjunction with CAPTCHA-style programs. xenForo, IPS are leading this charge to put a stop to forum spam with "background" mitigation that keep spam out. On top of that, there are also websites like WhatIsMyIPAddress that help forum owners decide whether the IP is spam, or not.
Also, Google and other major search engines are beginning to filter out "hits" from bots. So, your on-server analytics may look bigger than the ones found in Google Analytics, or similar... That's why, when you sell your websites and you need analytics to back up your claim, Google (or similar) is your friend.