WhatsApp is a high quality network and WhatsApp wants businesses to do their best to provide a good consumer experience. However, sometimes even the best of efforts of businesses is not enough to avoid being harassed by consumers in some form.
The WhatsApp for Business API and the CM.com business API do not currently support blocking incoming messages of consumers. You are however welcome to (automatically) ignore such messages in any software built on top of these solutions.
If a consumer is engaging in illegal behavior, please contact authorities.