The procedure is the same whether you use the Classic Exchange admin center or the new Exchange admin center.

Go to Mail flowRules – Create a new rule

Give the new rule a name and click on More options …

Click on Apply this rule if … and select The sender … and domain is

Add the domain you want to block.

Click on Do the following … and select Block the message …delete the message without notifiying anyone or one of the other two options regarding your needs.