As long as you are sure the devices that the ghosts were created from are unplugged/disabled, then in most cases you can remove the ghost via the process you've been attempting, using the hub's built-in tools.
See the info here:
Start w/this section: How do I remove Ghosts using the hubs built-in ghost removal tools? and follow steps in The best way to use the hub's built-in ghost removal tools.
If those steps don't work, then you will probably have to use a UZB stick (process described in the next section) to remove the ghost.
