If you have an account you would like to hide from certain parts of Monarch there are several options under the "edit account" menu that let you hide different parts of the account depending on what you are trying to do.
If the account is closed in real life its best to use the "close account" button which will hide the account from your list but preserve all its history.
The "edit account" menu is found as a button in the top right of the web app or in 3-dot overflow menu in the mobile app.