How to prove that a student is plagiarizing with paraphrasing software like Spinbot? Plagiarism software will not detect assignments that use Spinbot or other paraphrasing software. How do you prove a student is doing so?
I think you would need to know the particular source text that you believe it was modified from. In that case, demonstrating identical structure ought to be sufficient: a sentence-by-sentence comparison ought to show convincingly that the exact same ideas have been used in the same order. This absolutely falls under the definition of plagiarism (whether or not it was done in an automated way by a spinbot), and provided the text is of a decent length, it would be unreasonable for the student to claim that they happened upon the same structure by coincidence.

If you don't have the original text, I'm not aware of any tools you could use to search for it - that would seem to be a very complex (though maybe not insurmountable) AI problem. But if you don't know the source text, how would you suspect it in the first place (unless you'd caught the student in the act)?