I am 100% cretin that this is a vulnerability. The exact problem is "Client Side Trust" in that you are expecting the client to keep its state over a security system (your captcha). This is exploitable, a bot can just grab a new session with each request. You should be using a sql database to keep track of offenders based on their $_SERVER['REMOTE_ADDR'], and nothing else. Further more if you build security systems you MUST be responsible for when they end up on bugtraq.
