With wordpress i always set guest messages to be reviewed by an admin, though i know and heard there is a wide selection of spam plugins, i have used akismet which comes bundled with Wordpress and jetpack whichhas worked well for me but not all spam prevention stops everyone but we are getting better in fighting back.
I had to go with a captcha system for registrations as I was getting a large number of spam registrations. The moment I implemented it, I'm not getting any spam.
No, there is no 100 percent answer. Nonetheless, the free one I was using was good enough. I mean, everybody is always going to have to "clean house" a little on their own.To be honest, there are many plugins and services to deal with spam.
It's just that, they will never guarantee 100% spam prevention. The same goes to premium tools as well. Anyway, I use Cloudflare to make sure that not just anyone can access my website.
Also, I try to install good plugins as well and recruit people who are online most of the time, as staff members. In that way, we can deal with spam effectively.