If that Epic account isn't yours, then there's nothing you can do besides contacting Epic to see if they can do anything. They will, in this scenario, have permanent access to your account. If it is your account, then you need to resecure all platform accounts so that they cannot sign in to them and then they won't be able to play. Bungie will not forcibly unlink an account from a Bungie profile nor will they alter cross save setups to exclude accounts.