Hey there, this would only be possible for someone who knows your Epic account login (so not just your visible profile name, your actual login details) and password. You should change your Epic password and consider setting up two factor authentication, and consider finding out how someone got hold of your details, or your account will never be safe. Unfortunately if it's an account you can't authenticate, it can't be removed from cross-save. Bungie does not forcibly unlink accounts.