Just some words to the wise…as I see this type of situation occur nearly every day.
In 2018, segregation of your WordPress sites is one of the top 3 best keys to security.
Since it only takes one outdated “something” to allow hackers access to all of your websites, I strongly suggest you not host more than two WordPress sites on any shared account.
The more WordPress sites you host in a shared account the greater your chance you’ll be mass hacked in this way.
Analogy-wise, have you ever heard the term “Herding Cats?”
Because each WordPress installation is composed of thousands of “cats” the more cats you herd into the barn the harder it’s going to be to herd them back into the barn if they get out… At the moment your “cats” are currently scurrying about outside the barn. And you are attempting to herd them back in.
Once a site has been compromised, there are literally hundreds if not thousands of ways your hacker may be able to re-access your account. Simply changing passwords or removing obvious hacker users will not solve “the problem.” All are you doing is attempting to curb the symptoms of the attack. It’s likely they’ve placed backdoor scripts through your account at this point.
It might actually be smarter at this point to move some of your accounts off to a separate account, then harden each as best you can.
At least that way if your account is hacked again, you’ll have a better idea which one was the entry point, and you’ll reduce the likelihood all your sites will be hacked again.