You can do this using policy on the SMTP server.
- GateKeeper -> System -> SMTP Server -> Policies
- Set the default rights to 'are ignored'
- Remove any existing policies
- Click 'Add' to make a new policy
- Go to the 'Ban list' tab
- Select 'Enable ban list'
- Click 'Add'
- Select 'This criterion is met if SMTP Sender equals example@domain.com' (replace example@domain.com with an email address that you want to ban)
- Click OK
- Click OK
- Click Apply
You can block entire domains by changing the criterion slightly. e.g. 'This criterion is met if SMTP Sender ends with domain.com' or even 'This criterion is met if SMTP Sever contains domain'