Is it possible to limit the functionality that allows agents to ban visitors? Can I make sure that only Admins on the account can ban a visitor?
Yes! The permission to ban a visitor is managed in Chat roles. To make it so users with the Agent role can not ban visitors, from the Chat dashboard, go to Settings > Roles > Agent, and deselect the checkbox next to Manage visitor bans.
Learn more about chat roles and permissions in this article: How do I control what my chat agents have access to in the account?