This is not really something we random developers on here can handle - we don't work for Apple and have no access to their systems.
The only thing you can do with your Apple Developer account is remove the user so they don't have access.
What you need to do aside from that is make sure that developer has zero access to your code and systems. We don't know what systems you use, so that's for you to do, not us.
Topic:
Community
SubTopic:
Apple Developers
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