I have just finished developement of a captcha mod for traffic exchanges. This mod is an anti-cheat feature which will ask the user to input a 6 digit code every so many pages surfed. In the case of the LJ Script it will replace the surf verification feature that is default in that script, but may be turned off in the admin area (which will return your site to using the questions as before).
Captcha is a common turing test feature among many of the latest open-source scripts and e-commerce websites and is very efficient at preventing bots from using a particular service. If you’re worried about bot activity and would like to purchase this mod now please visit my Mods page. It costs $45 and can be running in as little as 15 minutes.