Post migration, is there a way to 'hide' accounts that I am unable to make inactive?

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Your financial reports in QuickBooks Online may contain information on deleted or inactive accounts. This occurs when a report retrieves information about transactions that were posted to the account while it was still active. Deleted accounts can be totally removed from reports. To do so, you may follow the instructions listed below:

To remove an inactive account from reports, you must make the account active first, then merge, move, or delete it.

Make an account active again.

1. Go to Settings ⚙ and select Chart of accounts.
2. At the top of the Chart of Accounts window, select the small Gear icon then mark Include inactive.
3. Find the account you want to reactivate, then select Make active.

The account will now appear in the Chart of Accounts window. We can now either merge the unwanted account with another one, or move the transactions to another account, then Delete.

Delete or move the transactions.

If you would rather change the transactions in the account and then delete it:

1. Look for the account you just made active. In the Action column, select Run report.
2. The Accounts QuickReport opens. Customize the Account QuickReport to All Dates, then select Run report.
3. Select each transaction, then either delete it or move it to a different account.
    

I've added these articles to learn more information on how accounts from the Chart of Accounts works:

• Understand the QuickBooks Chart of Accounts
• Manage default and special accounts in your chart of accounts

Post again here if you have other questions. I'm just a few clicks away to help you out!