Intuit does not allow you to delete customers/vendors/accounts etc.  It is a severe limitation of QBO.  Sadly, they could easily allow deletes, but they don't.  I don't know why, and they won't comment.  I do know that Intuit does not listen to its customers, and no matter how much you ask for this, or any change, they will do only what they want.  These forums are filled with customers begging for simple, logical changes year-over-year that never get fulfilled.

Note: Quickbooks Online is vastly inferior to Quickbooks Desktop, yet costs much more $$.  Consider switching to the desktop version, or picking a competing online bookkeeping software that values feedback from their customers - you'll likely save money too.