at the point of which they continue in such a way that is divisive what do we do?
What you can. Yet don't drop to their level. Act like the person you'd hope other people would be like. Don't sully yourself due to their intractable nature.
And you're right, I'd never thought about the fact that I could have "planted a seed." Change often does take time.
I think in any controversial subject or where you have someone acting in defense of their beliefs it almost always takes time. The cases where it does not are actually pretty rare.