If you wish to blacklist a particular prospect or even the entire domain from receiving emails in future, you can add them to Do not contact list.
First way: You can do this by going to the Prospects page in your account and search for the particular domain or prospect. Select the prospects and press "Add to Do not contact list" on the top.
Second way: You can also do this by going to Settings page in your account and go to "Do not contact list". Press Add button and type in the domains or emails you wish to add to your list.
Note that all these values should be separated by commas.