A long time ago I wrote about protecting contact_us.php with a simple Maths question. See http://www.clubosc.com/add-math-protection-to-contact_us-php.html
This has been possible in Phoenix for years, with no core code changes.
Just upload a hook file and your contact_us is protected.
I have just completed another way to protect your form, and this is available free of charge at the Phoenix Forum right here. This one implements hCaptcha, and is as easy as simply uploading a hook file.
If you would like to read more about hCaptcha (which is a solid replacement if you want to get away from the clutches of Google and its Recpatcha), you should have a good read here at hCaptcha).