I'm using the Contact Form 7 plugin on my website and I've also installed the Captcha, but I'm still getting many spam messages like this one: I also tested the captcha and it is working correctly. Do you know what can I do to prevent receiving these spam messages?
I faced the same problem, but once I installed the Honeypot plugin for Contact Form 7, all SPAM was eliminated: https://wordpress.org/plugins/contact-form-7-honeypot/ (It adds an invisible input field which must stay empty to submit the form. SPAM bots will fill it in so the form can't be submitted!)
The trouble with Captcha is that there are tons of jobs on the bidding sites for people to fill them out for pennies per thousand. The results of those jobs get pumped into spambots, so Captcha becomes completely and totally ineffective. That's neat! An alternative is the SOLVE Media Captcha plugin. You have to sign up with them (for free), but it also generates revenue (a few cents per thousand solves), so it's kinda neat. It integrates seamlessly with Contact Form 7 as well.