1. Decide which one you actually mean. Most people searching this want to stop the automatic bank feed, not erase the account from their books. These are genuinely different actions with different buttons.
2. To disconnect the live feed: go to Transactions. Click the tile for the account you want to stop syncing.
3. Open the account settings. Click the small pencil or settings icon on that tile.
4. Check "Disconnect this account on save." Confirm, then save. This stops future automatic downloads immediately; past transactions and the register stay exactly as they were.
5. To also remove it from the Chart of Accounts: go there next. Find the account, open the dropdown arrow in the Action column on the far right.
6. Delete it, if it genuinely has no history, or make it inactive if it does. QuickBooks only offers true deletion for an account that's never had a transaction posted to it. For everything else, which is almost every real bank account, Make inactive is the only option, and it's the right one: it hides the account going forward while keeping every past report accurate.