FRex, I meant the bots were configured by humans to answer the SFML question correctly. (Or maybe it's simple enough to be figured out by trial and error of smart bots...)
binary1248, but then you also exclude legitimate users (even with low probability). It might be a last resort, but so far the situation is not unbearable to require such strict measures. Making the registration process harder would also be a possibility, but that again annoys honest people. On the other hand, registering is only something you do once.