I will not use smtpd_sender_restrictions from Postfix, but create simple rule in MSH Traffic Policy.
1. The easiest way to do this is to create rule with block message action.
2. Choose REJECT option.
3. Now go to Conditions tab and define domains which you want to block. For this example I'll use *@gmail.com. Select Message Sender condition from the list.
4. Add domain gmail.com with asterisk character which will match any email address.
5. Select Message Recipient conditions from the list.
6. Add domain gmail.com with asterisk and choose TO,CC and BCC option from drop down menu - with this none email will go through this filter.
7. Remember to select Any condition must met - with this option when only one condition is met message will be dropped.
With that created rule no mail will go to or from gmail.com domain. There is no limit of defined domains, so adding new one is as easy as previous one.