Blocking phone numbers from the website on a line on my account

ATT secure family may be what you need. It's a subscription service 

To block specific numbers from your son's phone line on your account, log in to your account on the provider's website. Navigate to the section for managing phone settings or parental controls. Locate the option to block numbers and enter the numbers you wish to block. Save the changes to apply the blocking settings to your son's line.