You may want to add an email address to your suppression list.
To do this go to contacts > Suppressions. This will bring up the suppressions list for your account. To suppress an address at the top of the suppressions list, there will be a button that says ‘suppress a new address’
When you click this button, a new page will load.
On this screen, you can then add any suppression you manually want to add. You can even suppress a domain so that any email addresses on that domain will be suppressed. It is also advised that you add a note to put against the suppression. This is so when looking through your suppressions you know exactly why the address was suppressed.