How to block/redirect specific user agents

[subBlue]

Some of our sites have been crawled by the Indylib bot which has been spamming rubbish into URL parameters and generally being a bad bot. What is the best way to block requests from specific user agents?

The user agent header for the indy lib bot is:
Mozilla/3.0 (compatible; Indy Library)

[mistwang]

Use a rewrite rule testing ${USER_AGENT} should do the job.