Greetings!
Unfortunately, there is no way for you or Bungie to remove an account from your Cross Save setup without authenticating that you own that account. You will have to work with which ever platform your account is from. [quote]Players who can't authenticate connected platform accounts will have to contact their platform's support to try and gain access to the account. If that isn't successful, players may want to unlink their connected accounts they have access to and link them to another Bungie profile.
[u]Bungie is not able to remove connected platform accounts from profiles under any circumstances.[/u][/quote] via the [url=https://help.bungie.net/hc/en-us/articles/360049498031-Destiny-2-Cross-Save-Guide]Destiny 2: Cross Save Guide[/url].