Delete all outbound messages in queue. To do this you would go to the console page of the MTA and issue the CLEAR QUEUE Account# command. To clear the queue for the entire account issue the CLEAR QUEUE ALL command. You can hit the HELP button on the console page for more details on these and other commands.
Delete all outbound messages directing to a specific domain. To do this you would set up a delivery rule for the account or a global rule for all accounts. In the delivery rule settings create a domain-based rule for the domain and set the action to FAIL. Once this is set up, all outbound messages to that domain will fail.