When you delete an item from the library, all
instances or occurrences of that item in the document are also deleted
unless you specify that they not be.
Select the item and click the trash can icon at
the bottom of the Library panel.
In the warning box that appears, select Delete Symbol
Instances (the default) to delete the library item and all its instances.
Deselect the option to delete only the symbol, which leaves the
instances on the Stage.