I had a similar problem with a very good friend of mine.
He's a huge believer of most of the conspiracy theories floating around (including the flat earth theory) and he started sending me FB messages, with links, via FB Messenger. I told him I wasn't interested in his drivel, but he persisted - so I blocked him on Messenger. It keeps you as friends on FB, but blocks them from sending messages. He then started to write on my wall and tagging me .... so I just unfriended him.
I saw him the other week, for the first time in a while and said I'd add him again, as long as he didn't post any drivel on my wall. That was approx 4 weeks ago .... so far, so good :)