reCaptcha idea
Today google acquired recaptcha: http://googleblog.blogspot.com/2009/09/teaching-computers-to-read-google.html
This is, of course, a means to prevent bots from abusing websites intended for human consumption.
I think a clever way to continue fooling the bots would be to make letters made of letters. For instance, you might have a bunch of “t”s in the shape of an O.
t
t t
t
Such “global/local” tasks have proven difficult even for human beings on the autism spectrum, so I imagine it would be very hard for a computer.
In addition, in the instructions, you could choose to ask the user which letter (the compositional letter, or the macro letter) to identify. This choice could be made randomly, further stymying machines.