Steps to Block a Domain in Google Admin
Login to Google Admin Console
Go to admin.google.com and sign in with your admin account.
Navigate to Gmail Settings
From the Admin Console Home, go to:
Apps → Google Workspace → Gmail → Spam, Phishing, and Malware
Set up Blocked Senders
Scroll down to Blocked Senders section.
Click Configure (or Add Another Rule if one exists).
Add the Domain(s) to Block
Under Blocked senders list, enter the domain(s) you want to block.
You can add multiple domains, one per line.
Save & Apply Rule
Click Save.
At the bottom of the page, click Save again.
Note: Changes can take up to 24 hours to propagate.