Consider using a different device with a mobile internet connection THAT IS NOT CONNECTED TO YOUR WIFI/ROUTER. Borrow a friends phone or tablet and change your credentials on there. If it were me, I’d refresh my computer’s OS and get a different router. Hackers can compromise your router and filter the data packets for encrypted passwords and usernames and decrypt them on the fly. With this method you can change your credentials several times and he can capture the change each time.
It’s a lot of work, but if you want to be rid of this guy I’d try that. Oh and obviously a good malware and virus scanner. Those things are not guaranteed to work though.
Edit: It seems like other users are also complaining about there accounts being breached even after changing there credentials. It could also be an internal issue that Epic has to deal with. Beyond what I already wrote above, I’m not sure how else someone could so quickly get your password.