Procedure of site blocking with avast antivirus:
Open avast antivirus interface first.
Click additional protection menu in the left bar.
Then click on the menu site blocking.
You will see a page for site blocking. This enables you to block access to specific websites or URL.
Click on the checkbox enable site blocking.
Enter the URL or website you want to block.
Click ADD button below
URL or websites will be blocked and you will see a message when someone tries to access blocked URL or websites.
If you want to unblock, click delete button beside URL under site blocking menu.