If you need to quickly suspend e-mail for an e-mail address in cPanel and do not want to do this via the GUI, issue the following from command line as root:

whmapi1 suspend_outgoing_email user=$cpuser

Of course you’ll want to replace $cpuser with the username of the cpanel account you’d like to suspend e-mail for. The same thing goes for un-suspending:

whmapi1 suspend_outgoing_email user=$cpuser