
I changed the 'lang' variable that displays the error message: "The value of %s should be between %d and %d" to "The value of '%s' is wrong." so that bots wouldn't be given a clue as what to put.
Not the largest forum, but we went from 40 attempted spammer registrations a day to 1 or 2 per year.
This feature can be seen here: http://portlandkarting.com/forum/ucp.php?mode=register (after clicking "I Agree")