Typically we do not suspend an author's account unless we conclude they might themselves be associated with the chargeback activity, or conducting some other activity that is malicious, violates our Terms of Service, etc.
Really, the way that we handle preventing bad behaviour and chargebacks is via our royalty hold period and generous 45-day unconditional 100% refund policy. This helps reduce chargebacks, since readers have an easy two-click way of getting their money back if they are not satisfied. We find our books have a very low refund percentage, and we eat the 30 cent fee that refunds cost us. To learn more about our royalty hold period and our refund policy, see http://help.leanpub.com/author-help/leanpub-looks-interesting-but-why-is-there-a-45-day-royalty-hold.