[ocalhoun]

The solution is really quite easy.
In the template page that controls how the registration page will look, add a line that says 'enter the code you see here BACKWARDS' in nice big letters that even noobs can't miss, then in the php code that is behind that section, add a line just before the entered string is tested to see if it matches the original string such that the one of them is reversed (the strrev() function, I think).

Bots wouldn't be able to understand that they should enter it backwards, and they would all fail to register. (except for the very, very rare case of a random string which reads the same forward and backward)