You can go to the Chart of accounts and delete it from there if you're referring to deleting your current bank account. I'll show you how to do it.
Log in to your QuickBooks Online account.
Select Accounting menu, then Chart of accounts.
Find the bank account from the list.
Click the dropdown arrow under the Action column.
Choose Make inactive, then Yes to confirm.
If this is connected to online banking, you can disconnect it from the Banking page before making it inactive.
Here's how:
Go to the Banking menu.
Choose the bank account you want to disconnect.
Click the Edit pencil icon, then select Edit account info.
Checkmark the Disconnect this account on save checkbox.
Once done, hit Save and close.
However, if you're referring to your QuickBooks Online account. You can cancel your subscription at any time, and you'll have read-only access for a year after it deletes all of your data. Please visit this article for more details: Cancel your QuickBooks Online subscription.