brotherhood_of_LAN - 5:14 pm on Oct 30, 2013 (gmt 0)
I do a bit of browser automation and have to agree it's not as easy to block.
One thing that is worth measuring is the browser dimensions of the visitor. Another is to check whether an element is visible on the screen or not when 'something happens', i.e. if a submit button is pressed it should be visible on the screen.
Some of the bigger sites out there no doubt have experience of this as obvious IDs and classes in their DOM layout are obfuscated and randomised per page load, which makes it harder to grab hold of something on the page as a frame of reference to where everything is.