From your Dashboard click on the Gear Icon in the upper right, then choose Chart of Accounts

Find the account you want to Delete (it's actually made "Inactive" and just hidden), then in the Action Column on the right click on the little arrow for the Drop Down Menu.  Click on Delete and then Confirm.

Same basically for Classes.  Click on the Gear and pick All Lists then Classes.  Use the same Drop Down Menu to Delete (make Inactive) the Class you want to get rid of.