Currently each steam account is given a unique steam id number which is how most steam games identify the player and when you family share you are just associating that new steamid with your steamid so you can share certain purchases with if the developer allows it. Since each account is unique if I ban one it doesn't ban the other. In the past you could use the steam public web API to query a steamid to see if it was a family shared and it would respond with the parent account and you could compare that to your ban list and then ban the new account. A few years ago steam removed that capability for privacy protection and moved it to the game developers partner only access so a game developer could implement that same check but very few did and older or abandoned games are rife with cheaters now.

Now it would steam they are automagically making that check now or instead of a steam id it's a family id, I have no idea but if it prevents account whack-a-mole and brings back automation I'm all for it.