This is something I love about this forum.... the focus is properly placed on someone's behavior, rather than on their belief. Of course there is difficulty in defining the boundary and border lines that constitute 'left field' and to some extent even "Christian," but bad behavior anyone can recognize, no matter their belief system, sect or particular denomination (if they have one).
The idea that such people -- the ones who behave badly-- would be better off on another forum isn't necessarily a truism. The ones who behave badly here, likely behave badly wherever they go. That is, they behave similarly because of who and what they are. Put a '67 VW Bug in a new garage, it will still be an old Bug.
When banning decisions are based on beliefs rather than behavior however, suggesting that they should go elsewhere is a dangerous proposal. It's essentially telling people that if they believe differently than YOU, or than US-- they are not welcome here in our little 'Bible Club' and so we will take our "Club" in hand and chase you out of here. That's a very sad attitude and it's often the prelude to the death spiral of an online forum community. Or it turns a diverse community into an isolated sectarian compound or colony where everyone must conform or be 'excommunicated.' That's essentially how denominations, sects, and even cults operate. Fortunately,
@doughty has not been moving or moved in that direction.
I encourage you to continue operating in this spirit... A similar attitude by a Pharisee resulted in an untold number of people being given the opportunity to hear a soul-saving message in Acts 5.
Then he said to the council, “Men of Israel, pay close attention to what you are about to do to these men. For sometime ago Theudas rose up, claiming to be somebody, and about 400 men joined him. He was killed, and all who followed him were dispersed and nothing came of it. After him Judas the Galilean arose in the days of the census and incited people to follow him in revolt. He too was killed, and all who followed him were scattered. So in this case I say to you, stay away from these men and leave them alone because if this plan or this undertaking originates with people, it will come to nothing, but if it is from God, you will not be able to stop them, or you may even be found fighting against God.” He convinced them.
In other words, if someone's teachings are not of God-- they won't amount to much. If it is of God, you shouldn't stand in the way of it. Don't hinder, don't help. God can handle it.
A strange remorse. As a side note, it's often the behavior of those 'defenders of the faith' that is least Christ-like. Continue focusing on behavior and moderating that, rather than picking and choosing 'what kind of Christianity' is acceptable to discuss. This is meant as an encouragement to you and your team. I think you are doing a great job.