I have this exact same problem. I just redid my site after not touching it for a long time. I updated to WP 4.7.1 and the latest Fast Secure Contact Form from older versions and all of a sudden, after never getting spam, I’m getting over a dozen a day from mail.ru addresses.
I didn’t promote my site, link it up anywhere new, or anything to invite this. It literally started within a few hours of upgrading. If there isn’t a solution I’m afraid I’m going to have to go with a new contact form. I really wish the captcha strength was adjustable.
