You can set up a blocked senders list.
1. Sign in to the Google Admin console.
2. From the dashboard, go to Apps > Google Apps > Gmail > Advanced settings.
3. In the Organizations section, highlight your domain or the organizational unit for which you want to configure settings 
4. Scroll down to the Blocked senders section:
- If the setting's status is Not configured yet, click Configure (the "Add setting" dialog box displays).
- If the setting's status is Locally applied, click Edit to edit an existing setting (the "Edit setting" dialog box displays), or click Add another to add a new setting (the "Add setting" dialog box displays).
- If the setting’s status is Inherited, click View to view the inherited setting, or click Add another to add a new setting (the Add setting dialog box opens).
5. For additional instructions and guidelines, see the sections below. When you're finished making changes, click Add Setting or Save to close the dialog box.
6. Click Save changes at the bottom of the "Email settings" page.

Add addresses or domains to the list

After performing the steps above:

1. Click Use existing or create a new one.
2. Select the name of an existing list, or enter a custom name for a new list in the Create new list field, and then click Create.
3. Hover over the list name, and click Edit.
4. Click Add to add email addresses or domains to the list.
5. Enter the address or domain name, and click Save.
6. To add more email addresses or domains to the list, repeat steps 4 and 5.

When you're finished, click Add Setting (at the bottom of the dialog box), and then click Save changes (at the bottom of the "Email settings" page) to confirm your changes.