If you're trying to make sure your family member can't see a specific account, check out this article.
If you need to completely remove an account from Firstly, just head to the More tab on the mobile or web app, select Linked Accounts. From there, choose the specific bank. At the bottom of the accounts, you'll see Delete All Accounts for Institution.
You can always add an account back if you change your mind. But be aware that you may lose records of transactions older than 90 days on the Firstly software.