In any case, I don't mean to argue that anyone needs to waste their time on idiots. But ignore isn't a magic fix-all solution either and has in its own way cause a tremendous amount of harm, but sure it saves the poster from wasting their time. It can be easy to forget though that the most readers of many posts over the long run won't be regulars or thread participants, but randos doing research or people that landed in from a search engine.
Ignore is not a magical fix but at certain point you have to realize that you can never agree with some members and you will never be able change their mind.
After that point it perfectly makes sense to ignore that member, move on and stop wasting time with him.
You can however write any feedback you want on his profile, that can serve similar purpose as demeriting.