Hi

One option would be to format your PS4, which would remove all of the old owner's information. The other, delete only the users of the previous owner of the console. For the latter:

Turn on the PS4 and sign in with your main account. Then, go to Settings and select the Login Settings option. Here, select User Management.

Next, select Delete user. Find the user you want to remove and select it.

Press the X button on the Delete option to confirm.