Only two members were banned since my forum was founded in 2004.
One member got into a heated argument with two others about vitamins, and he contacted my fellow administrator, Saint, in private to demand a ban. Saint didn't take the demand seriously, and pointed out to the member that he could leave whenever he wants. Instead, that member said he would do everything he could to earn a ban, so Saint decided not to add to his workload.
Another member made it his mission, on Jesus' behalf, to undermine the forum. After a while, he got particularly irritable (e.g., starting sockpuppets to give him the means to make snark remarks about the forum, and posting replies just so he could make jabs at members), so staff decided to stop its source.
That said, the staff, including me, have never banned as a means to straighten out a member; it has always been a permanent last resort.