lucy24 - 1:12 am on Feb 27, 2012 (gmt 0)
If you don't care, send them packing to nowhere, a la:
RewriteRule .* http://127.0.0.1/ [L,R=301]
I did this for a while before realizing that they don't really go there, because robots have the option of not following redirects. But a 301 takes up even less room than a 403-- and some robots do slow down. (An earlier generation of my Ukrainians did.) Maybe they have to stop and figure out what to do next.
I've also seen the suggestion of redirecting them back to their own IP. Don't know what effect this would have if the source is an infected computer belonging to an unsuspecting human, rather than a genuine robot.