If someone is honestly mistaken about something the Bible teaches, and they are telling others what they think it says, are they a false teacher? It is true that they would be spreading a false teaching, but that doesn't necessarily make them a false teacher.
So what makes a false teacher a false teacher? See Am I Really a False Teacher? by George Sarris.
The man in the video below may fit the category of a false teacher. I can't see how he can honestly come to some of his conclusions. He has some nutty views and I don't agree with many of the things he says. His message in the following video is abrasive. It's so abrasive that I wasn't going to share it on this site. But I decided to because on this particular issue he makes some very important points that Christians ought to carefully consider. He certainly sums up what many people think (see the comments on YouTube).
What he says about this doctrine causing more people to reject God than all the wars and all the oppressive regimes combined may have some truth to it. Japan might have become a predominantly Christian country if it was not for that doctrine (see the video below). Not only has the doctrine caused many to doubt God's goodness, it has been the primary justification for persecution of heretics and Christians by other "Christians." See "A Legacy of Fear and Persecution" in Thomas Talbott's book The Inescapable Love of God.
Nowhere in the Bible does it say that a person has to accept a particular view about the nature, purpose and duration of hell to enter into God's Kingdom.
Perhaps you disagree with the video above. You may think universal reconciliation is a heresy that ought not be allowed in the church. If so, how should the church deal with those in the congregation who teach universal reconciliation? The church should deal with this the way Jesus taught us to deal with problems in the Church. See here.
With these things in mind, it's time we took a closer look at what the Bible says about hell.