Thanks for the suggestions.
Unfortunately, option 1 "Suspend Specific Mailbox in Reseller Account" stops the mailbox from receiving, but does not stop the server from handling the email.
The second option "Suspend Emails Services for a Specific Domain in Reseller Account" stops the server from handling the email, but unfortunately, it suspends Email Service for the Subscription, not a specific domain. So if my subscription include the domains mydomain.com and another.co.uk, I can suspend email services for both of them, not just one of them.