jcpunk

Where would you recommend I store this data?

I have a form mail system setup internally. It is as locked down as I can make it (only access is from inside, only able to email certain individuals, and a captacha). Users are complaining about the captacha. In the past we had a very smart spam bot infecting a computer that used the system to send spam. I intend not to be a spam gateway. So, is there any way (non session and non database) to say "Hey this user has visited this site within 300 seconds of their last visit, I wonder if they are a spam bot? Lets toss the captcha out there to find out." I cannot do session based stuff, because bots generally don't honor session cookies and that is what I am trying to prevent from using this. I don't want to put together a database because then I have to clean out old rows after my window expires. I am totally out of ideas on this one.... anybody got one?