If you only want to remove the bank accounts connected to online banking, you'll want to consider disconnecting them. The process will simply stop QuickBooks from downloading new transactions.
Click the pencil icon, and choose the Edit account info link.
Put a check mark on the Disconnect this account on save box. Note: If QuickBooks is downloading new transactions, you won't see this option. Wait a few minutes for the update to finish and try again.
Hit Save and Close.
I've got this sample screenshot for a visual guide: