I guess this is a difficult question. However, you can identify potential users to be removed. The ones that (AND):
1) Do not have any upvote (Do spams vote each other?)
2) Do not have any best answer (Do spams choose best answers of other spams?)
3) They have questions but they never chosen a best answer. (Do they return to choose the best answer?)
3) Do not have confirmed emails
4) Has a question or an answer identified as an spam (I am not sure how to do that, since I do not know how clever these robots are). Do the questions that they pose make any sense or follow any kind of pattern)? Is it possible to recognize the spam by the tags they use? Do they access your site just one time to register? Or they use the same account frequently? There are also some programs (mainly in English) that were designed to recognize spam.
So, my point is before you find an automatic way to recognize them, you have by yourself to be able to identify them.