Sorry for my english, I'm catalan,
I have had an idea, but my experience with Django is little for develop myself.
The principle is use a system of human verification with a image digits or any type of question/response.
In a middleware, this load a system of question/response (base class), and put into context of template a variable form for print with a question and response inside anything [HTML_REMOVED][HTML_REMOVED].
When the user post a form with his data and response verification, the middleware load response and question and send it to system question/response for validate the post data. Another form is use a decorator before view function for validate the response. Or use this as a simple function inside de view. (It can be configurable)
The limitation is that every time the middleware create a question/response and in a system with an image letters this can consume more time for create an image for question (except this use a set of know questions).
The advantage are: This can be tranparent to forms, views and models, and only afect to templates and view decorators. Other developers can write his systems question/response for adapt with his project.