You don't say what level your client is in their organisation. If, (as is generally the case with ineffective managers), they themselves have a boss, one of the best ways to address abusive behavior is to withdraw your services, thereby forcing your client's organisation to make a decision about whether to remove them, or force a change in their behavior.
The most important thing a consulting firm has is its reputation. Unsatisfied customers talk to potential customers and dissuade them from entering new contracts. There is a real financial cost to entertaining abusive customers- it is better for all concerned to enforce a code of conduct.