He was banned for making "ungodly and mean-spirited" comments about another speaker at the convention who believes the biblical story of the fall of Adam and Even can be viewed as an allegory. The speaker Ham spoke against is Peter Enns of the Biologos Foundation. Here's the description of the group from its website:
The BioLogos Foundation is a group of Christians, many of whom are professional scientists, biblical scholars, philosophers, theologians, pastors, and educators, who are concerned about the long history of disharmony between the findings of science and large sectors of the Christian faithHam is clearly an extremist, but it takes a special kind of extremist to attack people who are trying to promote harmony between groups who share sometimes conflicting views. That kind of extremism has lead to violence in the past. This convention was wise to ban Ken Ham.