Will did an excellent post a while back regarding Bot Trap. It is a honeypot that catches scraper bots and spam bots and then automatically bans their IP.
Since installing this on my blogs I've trapped hundreds of bots. It has not snagged any legitimate bots.
Bot Trap is free, easy to install, and is very reliable.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin