It looks to me like to make this all fit for you I'd have to deliniate my complete ecclesiology and my pastoral theology, both. :type: But I appreciate the questions. They help me articulate my position better. So I'll try to answer.
When someone from another church interacts with someone from my church, that is an inter-church matter and not yet heresy. The individual believer is a priest and can decide his own doctrine. However, as soon as that member of my church begins purposefully influencing other members of my church outside of the purview of his position in the church, it can become heresy, and that member is subject to church discipline.
So, in the case you are telling me about, yes, in the case of this post it has apparently become a matter of heresy, subject to church discipline.
I hope this helps. For a view similar to mine on ecclesiology, see Ecclesia, by B. H. Carroll. For a good discussion of church discipline that I like, see Principles and Practices for Baptist Churches, by Edward Hiscox, Ch. 7. Hiscox lists "false doctrine" as a cause for church discipline, defined as, "Holding and teaching doctrines fundamentally false, contrary to the law of God, as understood by the body, and subversive of their accepted faith" (p. 181).