keyplyr - 3:08 am on Apr 3, 2013 (gmt 0)
...scumbots mutate the UAs within seconds of receiving a 403.
Some do, not all... not even most. But that also seems to be a popular setting in some out-of-the-box scraper software. You can easily write a script to catch it if it's a consistent occurrence at your site(s).
Personally, I manually look through my logs each day (if not more) and visually catch these things, since these agents usually get caught doing other non-acceptable behaviors, then I give their IP address a look-see and if needed, block it for future.