What will Sony do if someone uses my account as primary on another ps4 without my knowledge?

The answer, sadly, is not much.

If your account was used on another PS4 and that PS4 was made into your account's primary PS4, you just have to wait. A PSN account can only have one active PS4, and you can only deactivate a PS4 by either having physical access to it or using your account-wide deactivation, which you can do once every 6 months. So for 6 months, the other PS4 will be your active one and you will not be able to use remote downloading. You can still purchase games through Sony's web store or the PS4 store, and install them normally. You won't be able to play these games (anything you do not own a disc for) without being connected to PSN.

When this happened to me I contacted Sony's customer support and they informed me that there was nothing they could do. They are not allowed to refund your account-wide deactivation, nor can they remotely deactivate a PS4. To my knowledge, Sony doesn't punish either party in these cases.

My advice is:

• Figure out when your PS4 was deactivated. Mark the date 6 months in the future as the time you will need to use your account wide deactivation.
• Change your PSN account credentials. I would highly recommend using a strong password (something generated by a secure source, not something you think up) and using a password manager to keep it safe. There are numerous options here and elaboration is outside of the scope of Arqade.
• If you used the same account name/password combination elsewhere (you should try to avoid doing this in general), change those as well.

If you remove a console as primary they aren't banned, they simply cannot access your PSN content unless they are connected to PSN at the time.