Unfamiliar wuth Gmail but don't these accounts usually allow you to set a sender, or even a domain as a spam source ? Check what is in the 'settings' menu, both generally and for individual received spam e-mails.
1. On your computer, go to Gmail.
2. Open the message.
3. In the top right, click 'More' (3 vertical dots next to the 'Reply' button, not the 'More' further up the page)
4. Click Block [sender].