I can help walk you through the steps on how to remove the bank account on your dashboard in QuickBooks Online.
You can make it as inactive in your Chart of Accounts. This way, the bank will not reflect anymore on your dashboard. No worries, your transactions are still in the account once you've made it as active. Here's how to do that:
Go to the Chart of Accounts and locate the bank you want to disconnect.
Click on the drop-down button beside View register.
Select Make inactive (reduce usage.)
Confirm it by clicking Yes. I've included some screenshots for your additional reference below.
If you want to make it active, you can simply go to the Cogwheel in the Chart of Accounts page and tick the box for Include inactive so that the account will appear in the list. Then, click Make as active along with the account name. If you want to connect your bank to the account, please see this article for more information: Connect bank and credit card accounts to QuickBooks Online. I've also added a screenshot where you can toggle those options I've mentioned about making your account active in connecting your bank.
Please don't hesitate to tag my name in the comment section below if you have other concerns. I'd be glad to help you. Have a wonderful day.