An easy way to avoid form-spam in the WordPress plugin Contact Form 7 is to add a quizz. Ask a question in order to sort out robots.
But don't ask questions like "what is five times five". The spambots are perfectly able to hack this sort of input. As an alternative - just ask something else.
You could add a picture to the web page and ask a question about it.
PS: I posted this four days ago, since then I had no spam mails via the mailform. So the quizz seems to do the trick.