Secure forms without using captcha

My web site uses an enquiry form that is sent to the owners of a property advertised on the site. I am currently using a captcha script to protect it from automated attempts at fraud from Africa. The robots auto complete the form reqeusting info about the property and promising to send payment for more than is asked. You know the kind of thing.

The captcha works fine. But I am convinced that it is stopping more people from making enquiries via the site. I hate them too.

So, does anyone know of a safe method to prevent this type of robot? They are not scanning for email addresses. Thanks
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