You're able to block entire domains, individual email addresses and mobile phone numbers from your account, by utilising the Block Contacts feature.
This feature is accessed from the 'account' option in the top right corner, then 'Blocked Contacts' in the horizontal nav bar.
When first using this feature you will be presented with this screen. Click the blue 'Block' button to begin.
In the following screen, you can type or paste in lists of domains, email addresses and mobile phone numbers.
When finished, click the blue 'Block' button in the lower right corner.
You will be shown a results screen afterwards.
Click the 'X' in the top right corner to return to the overview screen :
Here you can see a list of the blocked contacts for your account. Using the 'Trash' button on the right side of each contact, you can remove individual entries from the block list. You can do bulk actions by using the check boxes on the left and using the 'Actions' menu below.