auto update ip banlist from a file at cron

It would be good to auto update the list of banned ip's reading an external file of ip's at cron run.
I have some traps that I disallow in robots.txt and are very well hidden for real persons. SO if the robot hits the disallowed link.. I don't want that robot to be in my site so I redirect that link to a php where I add their ip to a file. That's the file to use at cron time to update Troll ip banning list.