You can manage your account via cPanel/WHM in Linux Reseller Hosting. WHM allows you to suspend/unsuspend your customers. So you can have full control over your customer. You can suspend the customer for the reason of billing or anything and enter the reason while suspending.During the suspension the domain will be redirected to cPanel default page instead of client index page. Once its fixed you can unsuspend easily.
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How do I Unsuspend a Domain Name under my Reseller Hosting Account ?