Accounts cannot be unlinked without being able to authenticate them. For security purposes Bungie cannot link or unlink accounts on behalf of players. Be aware, for that account to have been linked to your profile means that someone else knows either your XBox or your Steam credentials. You'll want to change your passwords immediately, and if you don't currently have two factor authentication turned on for those accounts you'll want to do that as well. If you are unable to remove the PSN account then your two choices are leave it there, which means you won't be able to use cross save and you won't be able to add another PSN account to this profile. Or you can move your Steam and XBox accounts to a new profile. Keep in mind that if you ever redeemed any emblem codes on the Bungie website they are tied to your Bungie profile which means they won't move to your new Bungie profile if you decide to create one. If you've never redeemed an emblem code on the website before, then you will lose nothing.

Only a person with access to at least one login linked in your profile can link an account. This means that if you didn't add it, someone else did. Unfortunately, Bungie cannot remove platform accounts from users' profiles for security reasons. Also, you will not be able to remove it unless you have the credentials or the person who linked it removes it.