There are two ways which you can blacklist a domain or email address so any emails coming from that user or domain will be moved into your spam folder.

Login to your webmail

The first option is to use the Spam option within webmail. 

Follow the steps below

1. Once logged in, select 'Settings' located at the top-right corner.
2. Select 'Email Accounts'
3. Click on the "Spam" tab along the top.
4. In the black list text box, enter the domain or email address. For example if I was going to enter a domain, it would be "example.co.nz". If I was going to enter a email address it would be "user@example.co.nz". Remember to replace example.co.nz and user with the actual domain and email user of the email you want to black list. 
5. Click on "Save"

The other option is to use the filters option.

1. Once logged in, select 'Settings' located at the top-right corner.
2. Select 'Email Accounts'.
3. Select 'Filters'.
4. Select '+Add'.
5. Under Filter Conditions, choose 'Select the conditions for your filter:'
6. From the drop-down menu, choose 'If any of these conditions are met'.
7. Choose Type 'From' - regex - matches - ^.+@.+.[domain]?$ (replace [domain] with the specific domain you want to blacklist, e.g., ^.+@.+.spamdomain.com?$)
8. Under Filter Actions, select 'Move to: INBOX.Spam'.
9. Press 'Save'.