A former member of your team is now using your Upwork username and password to work using your Upwork account?
You may need to work with Upwork Customer Support in order to resolve this. Have you created a Support ticket?
It looks like a member of the Customer Support Team is currently assisting you on your open support ticket. Feel free to follow up on the same ticket thread if you have further questions, or need clarification on your account and the team will assist you further.