Loomis can take an old account off your list and stop reading it. The account itself lives on the other site, so only that site can close or delete it.
Close it on the site first
If you want the account gone from the internet, sign in to that site and delete or deactivate it there. Loomis has no way to do this for you.
Release it from Your accounts
Your accounts comes with a Personal plan. On Your accounts, find the account's card and choose Release. Loomis stops reading it, and it no longer feeds your Insights. The card stays on the page, dimmed and marked Released, with a Restore button in case you change your mind.
If you connected the account, the card shows Disconnect to release instead. Choose Disconnect first, and then Release appears. A YouTube channel has only Disconnect, which does both at once.
Keep it but stop it counting
If you still use the account but don't want it shaping your Insights, turn off the Feeds Insights switch on its card. Loomis keeps it on your list.
If it was never yours
On Footprint, choose Not me on a profile that belongs to someone else, or I'm not on this platform if you've never had an account there. Loomis keeps those profiles out of your results and says nothing more about them.
For a supervised child
On a Family plan, a supervised child's Release goes to their parent first. The card shows Waiting on your parent until the parent answers.